For further reading on the forms. This seems pretty basic, but there are references to the works where he talks about this idea.
Scholars disagree about the scope of what is often called “the theory of Forms,” and question whether Plato began holding that there are only Forms for a small range of properties, such as tallness, equality, justice, beauty, and so on, and then widened the scope to include Forms corresponding to every term that can be applied to a multiplicity of instances.
The Wikipedia article goes much more in-depth, and includes criticism of the idea, from Plato himself. https://en.wikipedia.org/wiki/Theory_of_Forms
Finally, here is the Wikipedia article for Plato’s theory of knowledge, which I referenced in the last discussion. It contains reference to the Theory of Forms, but it doesn’t necessarily rely on it. I had confused the two during our discussion. https://en.wikipedia.org/wiki/Platonic_epistemology