Unenforceable means that a party cannot be legally forced to make an action by law, or by order of a court.
Unenforceable is the opposite of enforceable, which means that a party can be forced by law to do something or to make an action. Whether or not a contract is unenforceable can be very significant legally.
If a contract is unenforceable, it means that neither party is legally bound to the terms of the contract. (Handshake deals are often unenforceable.) However, if the contract is enforceable, then both parties involved in the contract are subject to the terms of the contract. This means that if one party does not meet his or her obligations in the contract, then he or she can be sued by the other party in the court of law. The court can then force the party to meet its obligations.