What os Definition of a Candidate's Move?

Candidate moves are moves that should be analyzed in further detail in order to determine what the best move is.

U mean most moves are candidate moves (of course except Blunders or obvious bad moves). Then we analyse those moves to find the best one?? Like every move with sign '?!' is a candidate move???

Candidate moves are moves which you analyze while playing. Naturally, you will get better at finding these moves as you improve at chess. Of course, we would always like to play the best moves, however we are not always capable of finding them. Typically, there are not so many candidate moves because as you mentioned, you would not analyse obvious blunders any further.

You usually select like 3 candidate moves: the most promising moves in the position. Then you calculate and evaluate each of them and then you select your intended move as the candidate move you have evaluated as the best one of your 3 candidate moves. Then you check your intended move is no blunder. Then you play it.