It's because you did not read the game rules gefore starting to play.
Oh you understood every thing about chess as a beginner? Wow, want a cookie?
That remark about the rules was not very good. You don't have to know all the rules, unless you are an arbiter. The FIDE Rulebook is several hundred pages long.
On the other hand, "check" and "checkmate" are absolutely essential. You simply can't play chess if you don't know what the goal is. It's more basic than how the pieces move.
What I was not understanding is that the opponent has to be currently in check. I thought because his next move would be checkmate, then it's done. But I get it now, Thanks so much