The German thinker was a giant of his time, and among his greatest contributions were his musings on the nature of knowledge. Basically, he's one of the strongest proponents of the "two worlds" of things, phenomenon and noumenon. Phenomenon describe things as we perceive them to be, while noumenon are the true nature of the things we perceive.
In Kant's view, phenomenon are the "wrappers" around the black box that is the "thing-in-itself". As human beings, limited by our physical senses, we will never be able to perceive the "thing-in-itself". What we know about the objects around us are limited to what we can perceive them to be.
Plato argued that an idea exists in its truest form as something in the mind of God. Take the divine idea of a chair. A carpenter is inspired by this divine idea to create the object that we know as the chair. An artist comes along and is so impressed by the chair that he makes a perfect painting of the chair. But, despite the painting's perfection, it remains no more than an imitation of an imitation of the true idea of a chair as it exists in God's mind.
The point that Plato was trying to make is that artists have a duty to be as good as they can be, because while they help to spread knowledge through their creations, they are already two steps removed from the "truth", and can risk further distorting knowledge if they aren't as accurate as possible in their depiction of an idea.