The computer determines what it regards as the best move and the likely continuation after that move and assigns the resulting position a numerical evaluation. By convention this is positive if White is better and negative if Black is better. So if, for example, White is up a Knight with best play from both sides, the evaluation us likely to be about +3.0 or so. Or if Black is up a pawn, the evaluation is likely to be about -1.0 (all other things being equal). Or if material is equal but White has a positional advantage the evaluation might be +1.5 or something.
The computer compares this evaluation with the evaluation if your move is played, followed by the likely continuation. If this evaluation for your move is the same as the evaluation following it's best move, then it is likely you made an excellent move. If it's nearly as good, your move is "good" - etc. If the evaluation of your move is 2 or so worse than the best move, it was likely a blunder.
Although it is true that the player might be planning something that the computer is not clever enough to see - perhaps because the point of the move only becomes apparent after the computer's calculated probable continuation - it is very unlikely. It can happen though, there are a few positions that computers don't understand properly, e.g. some fortresses.
However, the likelihood of this is extremely small - if the computer says a move was bad, then it was almost certainly bad, and you should try to find out why.
And I am a noob please don’t jump on me if that was a stupid question