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**
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> * 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).
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(you'll have to click Pennsylvania (PA) on the map to view it)