Some states have rules that allow any mail in ballot that is postmarked on or before election day; these ballots will not arrive to be counted until after election day, of course.

Also, mail in ballots are being used more by Democratic voters than GOP voters. And in record numbers due to the pandemic.

Trump is therefore incentivized to want counting of mail in ballots to cease as early as possible, to minimize a source of votes against him. His whining about when we will know the final outcome is designed to create pressure for that cessation of the counting.