This is also called a Platonic Form yes?
its hard to define, really. Lets start with this... all horses are, in theory, the same. they all have 4 legs, drink water, whatever, and we can identify them as horses. Plato said that a particular horse can "flow", like in time it will get old and die, but the "ideal" or "form" of Horse is eternal and immutable.
OR, you might think of it in terms of cookies, yum, cookies! If you see ten gingerbread men all sitting on the counter that all look the same, you have a subconscious understanding that they came from a cookie cutter, and what that cookie cutter is like. In Plato's world, that cookie cutter in the "ideal" of a gingerbread man.
I hope this helps.
Sophie's World by Jostein Gaarder