Complex sentence: Unification is an operation in computer science and logic which produces from two logic terms a substitution which either identifies the terms (in the case of syntactic unification) or makes the terms equal modulo some equational theory (in the case of semantic unification).
Split into simpler sentences: Unification is an operation in computer science and logic which produces a "binding" between two logic terms by which an identity relation is created. The binging is a substitution which either identifies the terms (in the case of syntactic unification) or makes the terms equal modulo some equational theory (in the case of semantic unification).