Well...this is a very general question. But I think one way to answer this question is that he was a rationalist. This means that he believed that ideas were very real, more real that objects themselves.
Plato believed in something call the "World of the Forms". He thought that physical things in the world were connected to their corresponding Form. To explain this, think about how you are always able to know that something is a chair. Even though you have never seen this particular chair before, you are not shocked by it. You do not walk into a class you have never been to and say "OMG, what are all these objects?". You know they are chairs.
Plato was very intriqued by this fact. We wondered how it was that we are able to do this. To explain how, Plato argued that our minds understand that FORM chair. Because we understand the very essence of a chair, we can apply this idea to particular objects. That is how I know a chair when I see one.