Universals are contrasted with individuals. 'Universal' used as an adjective is contrasted with particular and concrete.
Consider some examples of universals: there are types, like dog or "doghood"; properties, like red or redness; and relations, like betweenness or "being between"; those are all universals. Any particular dog, particular red thing, or particular object that is between other objects is not a universal, but a particular, and instances of universals (or objects that somehow bear universals). Doghood, redness, and betweenness are common to many different things. So a universal is something that can have instances; but it does not make sense to talk about an instance of a particular.
Realists invite us to think of universals as the referents of general terms. In other words, they are what we refer to, when we use general words like "doghood," "redness," and "betweenness." By contrast, we refer to particulars by using proper names, like "Fido," or definite descriptions that pick out just one thing, like "that apple on the table."
There is an ancient problem in metaphysics concerning what universals are supposed to be, or (alternatively characterized) whether they exist; this is called the problem of universals.