Some previous posters already got it, but just in case:
Plato says that the true reality is in non-physical, eternal, unchanging entities called Forms (or Ideas), and everything in the physical world that we live in is just an imperfect copy of these Forms. As in, every general concept like Beauty, Justice, Triangularity, has a perfect "Form", and these Forms exist independently of space and time, in a metaphysical realm. Physical objects are particulars that "participate in" or "imitate" the Forms, but never fully embody them.
Knowledge then comes not from sense experience (which according to Plato is unreliable and changeable), but from rational recollection of the Forms, and the soul knew the Forms prior to birth.
Sense perception only gives you opinion. Real knowledge(TM) is possible only through reasoning about the eternal Forms. The task of the philosopher is to turn away from appearances and apprehend the Forms through dialectic.
In other words, it's complete and utter sophist bullshit that has zero cognitive value.
Allow me to give you an example.
Greer v. Moon is not a mere legal dispute, it is a tragic reflection of the eternal Form of Litigiousness. Plaintiff Russell Greer does not act as a mushmouthed retard, but as the mortal conduit of the transcendent Idea of Petty Legal Wrangling. His motions, filings, and appeals are not strategic, they are the dim echos of Lawsuits That Always Were. Each docket entry is nothing but a shadow flickering on the cave wall of the court system.
Johm Potter, meanwhile, is not merely a defendant. He participates in the Ideal Form of Internet Anarchy, a chaotic spirit which radiates from the hyperuranion of Sneedposting. His resistance to judicial authority is not willful, it is metaphysically mandated by his essence.
The endless motions, the slander, the appeals to nonexistent detectives, the threats of bar complaints, all these things are not errors. They are necessary emanations from the eternal courtroom of Being, where every hearing is out of order, every witness is imaginary, and every Form is pro se.
Do you feel like this tale insults your intelligence, or is that just the Form of Insult manifesting imperfectly through syntax?
Did anything you have just read have any cognitive content, or was that just a shadow puppet show performed in Plato's epistemic basement?
If every courtroom dispute reflects the Form of Litigiousness, why bother with judges? Why not just consult the ether?
Are you thinking? Or are you merely recollecting thought-like shadows your soul glimpsed before you were born?
When you call someone or something beautiful, are you making a judgment or did your immortal essence just twitch in recognition?
If every spoon partakes in the Form of Spoon, can you eat soup with a Platonic ideal?
If a goat exists only by imperfectly instantiating Goatness, what does that make a goat-shaped pinata?
Does your coffee exist or is it just a pale reflection of the Eternal Brew?
Are concepts tools of cognition or do they hover in a metaphysical waiting room, hoping to be remembered?
If Plato is right and you forget all Forms at birth, how do you know that he remembered correctly?
Is a collapsed bookshelf just an imperfect realization of Order Becoming Disorder?
Can you stub your toe on an abstraction?
Why would one posit a second reality to explain the first reality, and then declare the second one "more real"?
If Forms are more perfect than reality, why do they require constant metaphysical babysitting?
Why does the idea of a perfect triangle need a cave allegory, a dialectic, and a soul whisper just to be noticed?
Is the "Form of the Chair" just Greek for "I don't want to define my terms"?
Can you cash the Form of Currency at the Form of a Bank?