The tenth amendment grants the states the ability to nullify federal laws that are unconstitutional if it chooses to but in the absence of an opposing state law that particular federal law is still enforceable simply because the mechanism to enforce it is there. This means that all federal laws are enforceable no matter if they are unconstitutional or not but nullification provides a check against unconstitutional powers. A state law that counters a federal law nullifies that federal law when that federal law is not a law that is in pursuance of the constitution or the constitution itself. Think of state and federal min wage laws. In some states state min wage is higher than federal and state law would be supreme in that situation because min wage laws are not one of the constitutionally delegated powers to federal government so state law kills federal law in that situation. However, a state law that attempts to regulate interstate commerce (which is trade between state borders) is inferior to federal law since the federal government has that power. This is how nullification works. It does not have the power to nullify constitutional federal laws. It only has the power to nullify unconstitutional laws of the federal government and even then it is the state's choice to do so. The state may like those laws and choose to keep them but in those situations it is the state's choice to keep them.