Unification is a fundamental concept in logic programming, and it is used in Prolog to match two terms. A term is a data structure that can be an atom, a variable, or a compound term. A compound term is a structure that consists of a function symbol and zero or more arguments.

The unification algorithm in Prolog works by recursively comparing the terms to be unified. If two terms are atoms, they are unified if they are the same atom. If one term is a variable and the other term is an atom, the variable is bound to the atom. If both terms are compound terms, the unification algorithm recursively compares the arguments of the two terms. If the arguments of the two terms are all unified, then the two compound terms are unified.

Unification is used in predicate matching in Prolog. A predicate is a clause that defines a relationship between two or more terms. For example, the predicate parent(X, Y) defines the relationship between a parent and a child. When Prolog is asked to prove a predicate, it uses unification to match the terms in the predicate with terms in the database. If the predicate can be unified with a term in the database, then the predicate is proved.

For example, the following query asks Prolog to prove the predicate parent(X, Y):

?- parent(X, Y).​

Prolog will first try to unify the term X with terms in the database. If it finds a match, it will then try to unify the term Y with the remaining terms in the database. If it finds a match for both X and Y, then the predicate parent(X, Y) is proved.

Unification is a powerful technique that allows Prolog to reason about relationships between terms. It is one of the key concepts that makes Prolog a powerful programming language.