The core of Plato's beliefs is that objects, properties and relations are merely copies of "Forms," which are non-material ideas that possess the highest and most fundamental kind of reality, as stated by PhilosophyBasics.com. This theory of Forms essentially states that intellectual truth is an unadulterated version of its physical imitation.
Plato's theory is controversial, because his belief these Forms are the only true source of legitimate knowledge call into question his own views, which are, by nature, rooted in reality. Plato's theory of Forms implies that grasping the world of Forms is the only way to gain true, pure intelligence. Plato also developed other theories to expand on the limitations of his theory of Forms.
The theory of Forms is just one part of Platonic realism. Platonic realism advocates the true existence of universals, meaning an object's properties can exist in multiple places at the same time. Because Plato considered universals to be an ideal form, this theory is sometimes called Platonic idealism.
Plato believed that while universals do exist, they are unrelated to space and time, and cannot be detected by known human senses, only conceived in the mind. Aristotle refuted Plato's claims, arguing that proof of universals and Forms rely on the prior knowledge of knowing what universals are to begin with.