This is a great question. Conceptualism is a philosophical view that states that the universality of particulars are frameworks in the human mind. If we fit it within philosophies, then conceptualism is in between the philosophical thoughts of realism and nomialism.
Let me give you a little historical context. In the Middle Ages, there was a debate of whether universals existed. Some stated that that universals did not exist and that it was only a human convention of language. There were nominalist.
Others stated that universal did exist. These followed the philosophy of Plato and they were called realists.
Still others took a more agnostic approach. They did not know whether these existed outside the mind, but they did know that they did exist at least in the mind. This is a conceptualism.
Conceptualism is a philosophical theory that explains universality of particulars as conceptualized frameworks situated within the thinking mind. Intermediate between Nominalism and Realism, the conceptualist view approaches the metaphysical concept of universals from a perspective that denies their presence in particulars outside of the mind's perception of them.