I will assume that you are talking about the F Stop as this usually causes most confusion.
The lower the F number the more light you are letting into your camera. Try to think of your F Stop as a set of venetian blinds. When the blinds are open you can only see the edge of them and they let lots light in the room (small amount of blind visible = small F number). When you start to close the blinds you can see more of them and the amount of light coming into the room is reduced (Large amount of blind visible = large F number).