Definition of gamble
A gamble is literally a bet. However, the term is a bit broader when applied to the world of poker. When we talk about a "gamble", we mean a somewhat risky play that may not be mathematically winning. Generally, professional poker players will make sure that all their games are EV+, i.e. mathematically winning in the long run. When you make a gamble, you close your eyes and pray.
Several types of gambles
There are several types of gambles. You can make a gamble for many reasons. In a tournament for example, we have four big blinds and it's folded to us. We're in the small blind and we have J4. Obviously, the hand is not great, but we have four big blinds. Here, it's possible to say, "I'm going for a gamble, I'm all-in and the die is cast". You can also be against an unknown opponent and push all in with a hand like 10-10. Against some players, the all in will be good. Against other players who are too tight, it will be a bad play because they will always have better. But against an unknown player, you can say "here, I gamble" and play your stack with a pair of 10's, without having much information about your opponent. In short, a gamble is a bet that you make without knowing your chances of winning.