Definition of probe bet
A probe bet is a bet out of position on the turn or river when a player has not made a continuation bet in position on the previous street. Do you follow? Let's take an example. A player opens on the button. The player in SB position calls the bet. The player on the BB folds. Usually, the player in SB position will check the flop to give the preflop raiser the initiative (otherwise his bet will be considered a donkbet). So the player in SB position checks, the player on the button does not bet and checks-back (miss-cbet). The turn is dealt and the player in SB position bets out of position: this is a probe bet.
More details on the probe bet
If a player bets in position without the lead after the player with the lead checks out of position (does not cbet), this bet is called a float. A float is a bet that is made after a check. This bet is made on the same street. So if the player who has the initiative checks out of position, the float will be made on the same street. But when the bet is made on the next street because the player who had the initiative was in position, we no longer speak of a float, but a probe bet.