A more thorough explanation:

Definition: Obstacle preemption is the principle that federal or state law can override or replace state or local law that creates a barrier to achieving the full purposes and objectives of the overriding federal or state law.

For example, if a state law prohibits the use of a certain type of pesticide, but a federal law allows it, the federal law would preempt the state law because the state law creates an obstacle to achieving the full purposes and objectives of the federal law.

Another example is the conflict between state and federal laws regarding marijuana. While some states have legalized marijuana for medical or recreational use, it is still illegal under federal law. In this case, federal law preempts state law because the state law creates an obstacle to achieving the full purposes and objectives of the federal law.