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. Typically, pro poker players will make sure that all of their plays are EV+, which means 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 4 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 4 big blinds. Here it is possible to say to yourself, "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 10s, without knowing much about your opponent. In short, a gamble is a bet that you make without knowing your chances of winning.