All-in is an act of betting all chips in your possession. It is possible to make an all-in during all betting rounds in the game, including the preflop. In No-Limit Holdem an all-in is used in the following situations:

You’ve collected the “premium” hand. Turn and River are the most suitable rounds for making an all-in – in later stages of the game opponents are more likely to call your bets. 

You have a good hand. If you’re sure your hand will result in a strong combination on the flop stage, it is wise to raise the bets. If the opponent calls very aggressively, it would be smart to make an all-in.

You have bad cards in your hand and want to bluff. Making an all-in while having a weak hand is a typical scene in movies, however, you shouldn’t repeat it in reality without necessity. 

You are in the tournament and have the biggest stack among the opponents. Your opponents with smaller stacks will do whatever it takes to stay in the tournament, so you make an all-in and add blinds and ante to your stack. Players with big stacks tend to play aggressively, while players with smaller stacks are more passive.

However, your opponents can also make an all-in. The smartest thing to do in this case is to simply look at your cards – you can safely call if you have a strong hand, and folding is the better option for those with weak hands. If multiple players make an all-in, the stack is split depending on the number of big blinds available to each player and the strength of the combination.