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