The supremacy clause says that:

Federal authority will replace state power when there is a legal conflict.

Explanation:

In United States , both the states and Federal government has the power to create their own legislations. Sometimes, the legislations that created by the states government can contradict the legislations created by the Federal government.

In order to handle this, The supremacy clause was added under the 2nd paragraph of the US constitution.

According to the supremacy clause, Federal government has more over authority compared to the states government. This means that if the states' law contradict the Federal law under a certain issue, the Federal law will be the one that will be implemented.