Father of my children died 2018, I have remarried. In 2023. Do I need to file form to explain why I am filing married filing separate since husband doesn’t have his W-2?

Not sure what you are trying to do.   There should not be anything about your late spouse entered on your 2023 return at all.   And if you are now married to someone else, that spouse is the legal step-parent of your children.   He has the same right to claim them as dependents on a tax return that you do.   

You could be filing a joint return  with your new spouse and claiming all the kids on that return.   When you file MFS you lose a lot of credits--especially child-related credits.

Just wait for his W-2 and enter all of the income on that one joint return.

If you were legally married at the end of 2023 your filing choices are married filing jointly or married filing separately.

Married Filing Jointly is usually better, even if one spouse had little or no income. When you file a joint return, you and your spouse will get the married filing jointly standard deduction of $27,700 (+$1500 for each spouse 65 or older)  You are eligible for more credits including education credits, earned income credit, child and dependent care credit, and a larger income limit to receive the child tax credit. 

If you choose to file married filing separately, both spouses have to file the same way—either you both itemize or you both use standard deduction. Your tax rate will be higher than on a joint return.

 Some of the special rules for filing separately include: you cannot get earned income credit, education credits, adoption credits, or deductions for student loan interest. A higher percent of your Social Security benefits may be taxable. Your limit for SALT (state and local taxes and sales tax) will be only $5000 per spouse. In many cases you will not be able to take the child and dependent care credit. The amount you can contribute to a retirement account will be affected. If you live in a community property state, you will be required to provide additional information regarding your spouse’s income. ( Community property states:  AZ, CA, ID, LA, NV, NM, TX, WA, WI)

 If  you are using online TurboTax to prepare your returns, you will need to prepare two separate returns and pay twice since with online, you get one return per fee.

https://ttlc.intuit.com/questions/1894449-married-filing-jointly-vs-married-filing-separately

https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states

https://ttlc.intuit.com/questions/1894449-is-it-better-for-a-married-couple-to-file-jointly-or-separately

Q.  Do I need to file form to explain why I am filing married filing separate?

A.  No.  If you want to file separate, you do not have to tell the IRS why.

But, you should tell you tax advisors  why you are doing that. Married Filing Jointly is usually the best way.  If the only reason is because you spouse's W-2 hasn't arrived yet, that's a very bad reason.  The IRS won't look at your return until Mid Feb.  Even if you never receive his W-2, there is a procedure  for filing a substitute W-2, but you have to wait until mid February. 

If you choose married filing separately as your filing status, the following special rules apply. Because of these special rules, you will usually pay more tax on a separate return than if you used another filing status that you qualify for.

1. Your tax rate generally will be higher than it would be on a joint return.
2. Your exemption amount for figuring the alternative minimum tax will be half that allowed to a joint return filer.
3. You cannot take the credit for child and dependent care expenses in most cases, and the amount that you can exclude from income under an employer's dependent care assistance program is limited to $2,500 (instead of $5,000 if you filed a joint return). For more information about these expenses, the credit, and the exclusion see Pub 17, Chapter 32.
4. You cannot take the earned income credit.
5. You cannot take the exclusion or credit for adoption expenses in most cases.
6. You cannot take the education credits (the American Opportunity credit and the lifetime learning credit), the deduction for student loan interest, or the tuition and fees deduction.
7. You cannot exclude any interest income from qualified U.S. savings bonds that you used for higher education expenses.
8. If you lived with your spouse at any time during the tax year:
a. You cannot claim the credit for the elderly or the disabled,
b. You will have to include in income more (up to 85%) of any social security or equivalent railroad retirement benefits you received, and
c. You cannot convert amounts from a traditional IRA into a Roth IRA.
9. The following deductions and credits are reduced at income levels that are half those for a joint return:
a. The child tax credit,
b. The retirement savings contributions credit,
c. Itemized deductions, and
d. The deduction for personal exemptions.
10. Your capital loss deduction limit is $1,500 (instead of $3,000 if you filed a joint return).
11. If your spouse itemizes deductions, you cannot claim the standard deduction. If you can claim the standard deduction, your basic standard deduction is half the amount allowed on a joint return.

You may not be able to deduct all or part of your contributions to a traditional IRA if you or your spouse were covered by an employee retirement plan at work during the year. Your deduction is reduced or eliminated if your income is more than a certain amount. This amount is much lower for married individuals who file separately and lived together at any time during the year.

If you actively participated in a passive rental real estate activity that produced a loss, you generally can deduct the loss from your non-passive income, up to $25,000. This is called a special allowance. However, married persons filing separate returns who lived together at any time during the year cannot claim this special allowance. Married persons filing separate returns who lived apart at all times during the year are each allowed a $12,500 maximum special allowance for losses from passive real estate activities.

If you live in Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington or Wisconsin and file separately, your income may be considered separate income or community income for income tax purposes. See Pub 555 Community Property - http://www.irs.gov/pub/irs-pdf/p555.pdf The states of Tennessee and South Dakota have passed elective Community

Property Laws.