Ok ... you did not make any estimated quarterly payments then you can skip this section.
However if you did (and you would know that you did) then of the total payments made you are to enter how much of the 2018 estimated payments were made in 2019 ( the 4th 2018 estimate was due 1/15/19 however you could have paid it early in 2018) ... look at your records for cancelled checks or CC statements.