It's hard to summarise. Read "Introduction to Objectivist Epistemology" which contains the fundamentals of a theory of knowledge, specifically a theory of the nature of concepts. That's the part which is relevant for this discussion and is also the foundation for everything else.

Ultimately you have to read Rand rather than read about her, as so many of her critics enjoy a somewhat loose relationship with reality. For example, the comment in this thread about "randian evangelism" on HN. I've encountered two other Objectivists on HN, whereas every week we have multiple hundred-comment threads on how fantastic an idea universal basic income is. That's pretty ineffective evangelism.

There's enough randian evangelism on this forum without inviting more.