The first two paragraphs of the home page say what it is:

MiniZinc is a free and open-source constraint modeling language.

You can use MiniZinc to model constraint satisfaction and optimization problems in a high-level, solver-independent way, taking advantage of a large library of pre-defined constraints. Your model is then compiled into FlatZinc, a solver input language that is understood by a wide range of solvers.

You forgot to look up the trivially googleable term (or to read TFA) before deciding to post.