I'm doing my form SSa1099. There's a question "did you receive a lumpsum payment in from 2015 or prior?" Are we talking about money from an underpayment from a prev year

Yes, it could be from an underpayment in a previous year.  Or, sometimes it may be a retroactive payment for prior years when Social Security benefits are just getting started.

There should be an amount listed on your Form SSA-1099 in box 3 with an asterisk (*) beside the dollar amount.  Then, at the bottom of the page in the box showing “Description of Amount in Box 3”, there is an explanation for the previous year amounts paid.