What is rmd and how to calculate?

The plan administrator should calculate it for you.  What type of plan?

Virtually all pension plans, defined contribution plans, and annuities in existence today fall under section 401 of the tax code.

Required minimum distributions for defined benefit plans and annuity contracts all require RMD's after age 70 1/2.  If you receive periodic (monthly) payments from any pension plan you can be assured that the custodian of the plan is meeting the requirements so you can safely answer "yes" to the question.

The rules that the plan administrator must use to calculate the RMD as required by §401(a)(9) are defined in § 1.401(a)(9)-6) of the Internal Revenue Code (IRC).

The answer to the question does not go on your tax return, it is simply to make sure you do not owe any penalty for not taking the RMD or use it for an ineligible rollover.

The question is just to be sure that the requirement was met.

https://ttlc.intuit.com/replies/3302117

Required Minimum Distributions are required on pensions and IRAs  and they are calculated for you by the plan administrator.