Daughter is 40. Lives with us and is disabled. How do I explain that we help her out?

There are no explanations that you can include with your tax return. If she qualifies as a dependent, you can include her as such.

Qualifying Child

 Relationship — the taxpayer’s child or stepchild (whether by blood or adoption), foster child, sibling or step-sibling, or a descendant of one of these.

Residence — has the same principal residence as the taxpayer for more than half the tax year. Exceptions apply, in certain cases, for children of divorced or separated parents, kidnapped children, temporary absences, and for children who were born or died during the year.

Age — must be under the age of 19 at the end of the tax year, or under the age of 24 if a full-time student for at least five months of the year, or be permanently and totally disabled at any time during the year.

Support — did not provide more than one-half of his/her own support for the year.

Qualifying Relative

You can claim a child, relative, friend, fiancé (etc.) as a dependent on your 2019 taxes as long as they meet all of the following requirements :

• You provided more than half of their financial support. More info
• They made less than $4,200 in gross income during 2019 unless they are a qualifying child.
• They live with you or they are related to you. (Your relative must live at your residence all year or be on the list of “relatives who do not live with you” in Publication 501.) 
• They are a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren't (or won't be) claimed as a dependent by someone else.
• They aren’t filing a joint return with their spouse.
• You are not being claimed as a dependent on someone else's return.

@bellefrosty2

"Helping her out" isn't enough for taxes, so there is nothing to explain. 

Either she qualifies as your dependent, or she doesn't. There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test. Only a QC qualifies a taxpayer for the Earned Income Credit.

A child of a taxpayer can still be a “Qualifying Child” (QC) dependent, regardless of his/her income, if:

1. He is under age 19, or under 24 if a full time student for at least 5 months of the year, or is totally & permanently disabled
2. He did not provide more than 1/2 his own support. SSI (is  considered third party support (welfare) and not as support provided by the child.  SSDI is considered the child'sown support. 
3. He lived with the parent (including temporary absences such as away at school) for more than half the year

A person can still be a Qualifying relative dependent, if not a Qualifying Child, if he meets the 6 tests for claiming a dependent:

1. Closely Related OR live with the taxpayer ALL year   
2. His/her gross taxable income for the year must be less than $4200 ($4150 in 2018)
3. The taxpayer must have provided more than 1/2 his support

In either case:

4. He must be a US citizen or resident of the US, Canada or Mexico
5. He must not file a joint return with his spouse or be claiming a dependent of his own
6. He must not be the qualifying child of another taxpayer

Full rules at 

https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Return/INF12139.html