The above post has given you a great definition and examples of economic equilibrium. This is an economic situation that is probably rarely achieved and when it is we probably don't stay that way for a very long period.
Equilibrium is an economic term that refers to a situation where there is an exact match between how much of something people want to buy and how much other people want to sell. Equilibrium occurs at some given price -- at that price, the amount of the good or service that people are willing to buy is exactly equal to the amount that producers are willing to sell.
At equilibrium, you do not have a shortage (people want to buy, but there are no more to be bought) or a surplus (suppliers want to sell, but people don't want to buy).