It means that the vote counts of those 700,000 ballots weren't allowed to be observed by some Republicans (they were denied entry, because they didn't register upfront and/or were misbehaving), and by Trump's reasoning, those votes should not count. Those votes were mail-in ballots in urban areas, and most of them went to his opponent Joe Biden. Without counting those votes, he might have had the majority of votes in Pennsylvania.
However, the law that governs this is not the Constitution, but in this case Pennsylvania Statutes Title 25 P.S. Elections & Electoral Districts § 3146.8. Canvassing of official absentee ballots which the National Conference of State Legislatures summarizes as
Absentee ballot processing and counting
- Partisan observers are permitted to be present when absentee and mail-in ballot envelopes are opened, and when the ballots are counted and recorded (25 P.S. § 3146.8).
(you'll have to click Pennsylvania (PA) on the map to view it)