Checkmate without check beforehand [Title &quot;fucccommies-Diam0ndJake, chess.com, 7/8/2020&quot;]
[FEN &quot;&quot;]

1. e4 e5 2. Nf3 Nf6 3. d4 Nc6 4. d5 Nd4 5. c3 Nxf3+ 6. Qxf3 Be7 7. Bb5 O-O 8. O-O c6 9. dxc6 bxc6 10. Bc4 d5 11. exd5 cxd5 12. Rd1 Qb6 13. Be3 Qb7 14. Bxd5 Nxd5 15. Rxd5 g6 16. Na3 Bxa3 17. bxa3 Qb2 18. Rad1 Qxa2 19. Ra5 Qb3 20. Rxe5 Bb7 21. Qf6 Qxd1#

In this game, I which my opponent and I played poorly, I ended up winning without a check at all before the checkmate. Can anyone help me understand when checkmates can happen without a check?
Assuming you are Black, you have checked your opponent - your Queen on d1 is putting his King on g1 in check (and checkmate).
Edit: assuming Rewan Demontay's reading of your question is correct, then there's no stipulation that you have to check your opponent before checkmate. You can see this in well-known short checkmates, e.g. Scholar's mate. 1. e4 e5 2. Bc4 Bc5 3. Qh5 Nf6?? 4. Qx7# is checkmate without checking first.