@EdRick - Imagine that a virus is wearing a nametag. The nametag is its antigen, its identifier. Then the vaccine is like a picture of that nametag. Imagine the vaccine showing that picture to the immune system and saying "Hey, if this guy shows up, show him to the door."
Then, when the virus shows up with his nametag on, the immune system jumps right on him.
But if you have not been vaccinated, the immune system hasn't seen that picture (i.e., has not been exposed to the antigen before). It has to wait for the virus to cause trouble before it knows what to do. Now, then if you get the same virus again, the immune recognizes the antigen and leaps right into action so you don't get sick.