Stalemate?

I'm a new chess player, so I may be missing something but here goes:

(Analysis Link):  https://www.chess.com/analysis/game/live/5646759570?tab=analysis

After a very tense game, I make a breakthrough and win a ton of material including the opponents queen. The final board looks like this

At this time I'm very excited because I played a very good game (for my level) and I was one step from checkmating my opponent (I haven't moved my rook to a7 yet). As soon as I move my rook to a7, the game ends in a draw. The draw is instant, there's not even an accept button or anything. Just....draw. Is there a rule I'm missing, or a game mechanic? Why was it a draw?

Stalemate.

Wikepedia would explain it better than I could

https://www.google.com.au/url?sa=t&source=web&rct=j&url=https://en.m.wikipedia.org/wiki/Stalemate&ved=2ahUKEwit6eydytbsAhUo4nMBHRjpBuwQFjAXegQIEBAE&usg=AOvVaw36fcvhele70I4r1Axfrm0N

It's a draw because your opponent has no legal move and is not in check.