Going, "All In" is a Texas Holdem term meaning that you are betting your entire stack of chips on this one hand. In a tournament, it may be your last hand if you loose. Some tournaments allow you to rebuy chips. At a brick and mortar casino in a regular Holdem Poker game you can rebuy as many times as you can afford
Going All In can be a defensive move. Suppose you have a very good hand but do not have the chips in front of you to keep up with the betting. Going all in in this case will let you share that part of the pot in which you already have money invested.
Going All In is the strongest bluff. Nothing talks like money nor is anything more convincing in a poker game than putting your whole stack up against the out come. Try this tactic to much and you are going to loose Big Time. There are very few shrinking violets playing poker. It is a gamblers game that demands a high degree of skill. Trying to bluff by going all in over and over again is going to get your bluff called by more than on poker playing gambling man at the table.