"Field preemption" is a type of federal preemption of state law. It's often found when federal law occupies a field of law so completely that it's reasonable to infer Congress left no room for states to regulate concurrently. (See, for example, Pennsylvania v. Nelson, 350 U.S. 497 (1956) (federal law completely occupied the field of criminal sedition, leaving no room for states to regulate).)
Unlike most kinds of federal preemption, which can be found only when there's a conflict between federal and state law, a finding of conflict isn't necessary for field preemption. In other words, even state laws that are consistent with federal law can be preempted on a finding that federal law occupies the field.
Courts also use field preemption under state law, to find that local laws are preempted by state-level regulations.