A more thorough explanation:

The extra-elements test is a legal test used to determine whether a state-law claim is preempted by federal intellectual-property statutes under the Sears-Compco doctrine. This test is used to determine whether a state law claim is different enough from a federal infringement claim to be allowed to proceed.

For example, if someone sues for copyright infringement under state law, the extra-elements test would be used to determine whether the state law claim is different enough from a federal copyright infringement claim to be allowed to proceed. If the state law claim requires proof of an extra element that makes the action qualitatively different from an infringement action based on federal law, the state action is not preempted.

Another example would be if someone sues for trademark infringement under state law. The extra-elements test would be used to determine whether the state law claim is different enough from a federal trademark infringement claim to be allowed to proceed. If the state law claim requires proof of an extra element that makes the action qualitatively different from an infringement action based on federal law, the state action is not preempted.