In this sentence lawmaker is a compound noun. This type of noun surge from a combination of two words. They commonly have two parts, the first that tells us the kind of object of person it is while the second identifies the object or person in question. Basically, first part says the type or purpose, the second one what or who. Combined they can give a more specific meaning than the two separate words. In this case we understand that a lawmaker is who writes the laws, literally who makes the laws.
