A Severability clause states what will happen to an agreement if part of that agreement is declared unenforceable by a court.
Some alternatives stipulate that the potentially unenforceable clause may be rewritten to be enforceable (Rule of Reasonableness). Others allow the clause in question to be rewritten unless the clause addresses an "Essential Purpose" of the agreement. If the clause addresses an Essential Purpose, the clause should not be rewritten and the entire agreement should be held unenforceable.
The Severability clause generally contains two parts, (a) savings language to preserve the remaining agreement in the event a court finds a part to be unenforceable, and (b) reformation language that describes how the parties intend unenforceable parts to be modified to be enforceable, or simply deleted.