Each protein contains a different levels of the 9 essential amino acids you need. If you use protein from a source like dairy, eggs or beef you don't need to worry too much about the balance of these amino acids. This is because these animal based proteins contain contain good levels of all amino acids your body needs. These are generally referred to as complete protein sources because of this good balance.
However, if you follow a vegan diet things get a little bit more complicated. While plant-based proteins can be very high in certain essential amino acids, few plant-based sources have good levels of all the essential amino acids. In fact, the only vegan source with decent levels of all amino acids is soy, though we generally don't recommend a high soy diet.
So how do you get a good balance of essential amino acids with plant protein? The solution is to mix plant-based proteins that complement the deficiencies of each source. A common example is a blend of both rice and pea protein which complete the inadequacies of each and together provide a complete protein source.