Thus we may conceive how words, which were by nature so well adapted to that purpose, came to be made use of by men, as the signs of their ideas.

This is a concise statement of Locke's theory of meaning. Words, in his view, do not refer to things in the external world, but to the ideas in our head. When you say "dog," for instance, you are not really referring to any dog out in the world, you are referring to an idea you have in your mind of something furry, four legged, loyal, and panting. Even if you say "Lassie" you are not referring directly to that creature you see running around the television screen. You are referring to the idea you have of the dog running around the television screen, in this case the idea that is, or once was, your sensation of that particular dog.