Better explained by KnowYourMeme, but it's basically pressing and never releasing button A.

https://knowyourmeme.com/memes/05x-a-presses-but-first-we-ne...

Half presses aren't counted as part of the record (it is a whole) but do matter when doing multiple levels in sequence as you can often not spend an extra one if you had a "half" from the one before.