When you gamble, you take a risky chance on something. When an entrepreneur invests her own money in her business, she gambles on its success.
If you approach a stray dog, you gamble on its friendliness, and if you turn down a high-paying job to take a more interesting position, you gamble on it eventually paying off. Another way to gamble is to buy a lottery ticket or place a bet on a poker hand: when you gamble like this, you play a game of chance, risking some amount of money. Gamble is from Old English gammlen, "to play, jest, or be merry."