What is a quotient structure?

Question

I was reading a paper here, and it mentioned "quotient structure" in the following sentence (third page, second paragraph of the paper)

In order to obtain a representation of terms truly isomorphic to λ-terms, we need to build a quotient structure, quotient the set of raw terms with respect to alpha-equivalence. This construction based on a quotient corresponds very closely to the of presentation from standard textbooks on λ-calculus.

I am hoping that someone could point me a good tutorial on quotient structures.