In Plato's Theory of Forms (or "World of Ideas"), knowledge is justified, true belief. That is, one could believe in the Truth but not have a justified reason for it, more an accident than anything (or one was persuaded without reason). Any other combination of belief is therefore wrong.
We must, therefore, carefully use our reasoning, our minds, to discover evidence that we are perceiving reality correctly. For ideas that do not have an objective or physical reality, our minds can still reason to discover Truth.
Plato's famous example of the ur-Horse reflects the concept that mortal flawed ideas are reflections of the immortal Idea. In other words, we see many flawed mortal world examples of horses and the good, but each of these is evidence to what the ultimate Horse (ur-) and Good actually are.