For Plato there are really two worlds in existence -- the real
and the actual.
When you look at the chair you were just sitting in, you are not seeing a real
chair, just the actualization of the ideal of chairness.
What you perceive through your senses, for Plato, is just a form of the real
world, which is perfection itself but not available to us poor humans on a sensuous level.
article which dale linked is okay, but it makes a fairly simple concept sound unnecessarily complex.)