It means what it says
The person who owns the land has a right to a seat in the parish church and, if there are not enough seats then they get one and other people have to stand.
This is all pretty archaic but it stems from English law where parishes were geo-political and not just religious. Who got to sit was decided by the church-wardens and parishioners had a right to a seat without payment - visitors could be charged. However, some parishioners had an additional right by virtue of their office or landholding to a seat before other parishioners.
The United States is in some ways a legal “time capsule”. Many common law countries have progressively codified the common law which tends to “fossilise” the law since judges are no longer able to say “well, that was a sensible law then but it’s clearly outdated so I’m changing it”. Since the United States did this earlier and harder than most other jurisdictions and has a natural progression back in time from the west to the east you get these lovely little anachronisms.