Definition of gamble
An gamble is literally a bet. However, the term is a little broader when applied to the world of poker. When we talk about an "gamble", we mean a slightly risky game that may not be mathematically a winner. Typically, pro poker players will make sure that all their games are EV+, which means mathematically winning in the long run. When you roll an gamble, it means closing your eyes and praying.
Several types of gambles
There are several types of gambles. You can make an gamble for several reasons. In a tournament for example, we have 4 big blindss and it's folded up to us. We are on the small blind and we have J4. Obviously, the hand is not famous, but we have 4 big blindss. Here, it's possible to say, "I'm going for an 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 game 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 too much information about your opponent. In short, an gamble is a bet we make without knowing our chances of winning it.