Answer:
To Plato, ideas or forms were the abstract true shapes of the everyday things that we can directly perceive.
Explanatio:
These ideas or forms exist in a separate reality called "the world of ideas", which we cannot see. This is why Plato is labeled an "idealist", and many philosphers after him who agreed with him became part of the idealist branch of philosophy, as opposed to the materialist branch.
Plato illustrates his point with allegory of the cavern: a person lives in a cave and perceives a shape. He thinks the shape is real, but it is actually a reflection of a sort of toy that represents the shape, a toy that is being held and shown by another cave dweller who has access to fire. Thus, the first cave dweller (us) cannot see things for what they really are, only for what he perceives they are.
I will provide a picture of the allegory below that dispays this idea better.