When you can make people obey a rule, it's enforceable. Banning phones from school might not be enforceable, but most teachers are able to keep students from openly using them in class.
When a law isn't enforceable, it's because it's impossible to make people comply with it (like a law saying you must smile all day long), or because everyone agrees it's ridiculous (like a real law in Georgia that makes it a crime to eat fried chicken with a fork). To enforce a rule, law, or contract is to actually carry it out by compelling people to follow it. So anything enforceable makes this possible.