I believe he's referring to mail-in ballots:

3 day extension for Pennsylvania is a disaster for our Nation, and for Pennsylvania itself. The Democrats are trying to steal this Election. We have to get out and VOTE in even larger numbers. The Great Red Wave is coming!!!

That tweet is about PA's move from "received on or before the 3rd" to "mailed on or before the 3rd at 8 PM and received by the 6th at 5 PM." The problem is, they didn't limit the definition of "mailed by" to "postmarked by;" they also included "ballots with no postmark, or with an illegible postmark, but with no clear evidence that they were mailed after the 3rd" (note: paraphrased). The first part alone would be fine, but that last bit could jeopardize at least some ballots that were sent on time. Supposedly, late-arriving ballots will be set aside, but if the ones not clearly postmarked by the 3rd are not separated from those that are, all of them may be thrown out.

I'm hoping there's a large enough margin from ballots that did arrive by today that all late-arriving ones are inconsequential.