To float in poker means to call a bet without a made hand or a strong draw with the intention of taking over the betting lead if your opponent shows a sign of weakness.
For example, you call a preflop raise on the button with JhTh and the flop comes 2s4s9h. Your opponent bets, and you decide to call even though you don’t have a pair or a direct draw.
This call is a float, as you’re not only looking to improve but also win the pot if your opponent gives up a betting lead on the turn. For example, the turn comes another 4, and they now check. You can now bet to try and win the pot by sheer aggression despite your actual hand not improving.