A postulate is basically a hypothesis. In other words, it's a very strong statement. Typically when we think about a postulate we also call it an axiom. An axiom is typically something that needs no proof. For example, between two points there is a line. Well obviously if I draw two points on a paper and connect them it creates a line.
A postulate is similar in that it's a statement that I don't need to prove because it's so obvious. But as you get farther and farther into geometry in particular, those postulates becomes less obvious so you still need to understand how to prove them.