What does 'mate in one' mean in chess? Apparently, I've done it

I was randomly playing against chess.com's top level bot and it made this blunder. No clue how. (Game link is in comments) : r/chessbeginners

Checkmate - Wikipedia

New chess rule #chess #chesstok, chess edit

WHITE to move and MATE IN 3 : r/chess

Reminder: “When you see a good move, look for a better one.” Missed mate in 2. : r/chess

Need help! : r/chess

My Opponent resigned, Can someone explain to me why this wasn't a checkmate : r/chessbeginners

What's the easiest way to win chess in as few moves as possible? - Quora

Check, Checkmate, & Stalemate Differences Explained (with GIFs)

A cool tactic I found today - white to play and win! : r/chess

What is a mate-in-one in chess, and how rare are they? - Quora

Mate in 5 I saw while watching a game between GMs (the GM didn't see, game in the first comment) : r/chess

What does 'mate in one' mean in chess? Apparently, I've done it many times since my buddy keeps telling me it's my favorite checkmate move? - Quora