It seems there is no problem thinking about what triangles and white things are, but what of triangularity and whiteness?
Realism asserts that universals are real things, that triangularity does refer to something in the world, although perhaps some metaphysical realm.
Plato is an example.
There are no abstract ideas, just particulars.
One can accept that universals refer to things in the world but be clear that they are mental things (concepts). Locke is an example.