A more thorough explanation:

When Congress passes a federal law that is so comprehensive that it leaves no room for state law in a particular area, it is said to "occupy the field." This means that the federal government's authority in that area is dominant, and state laws on the same subject will not be enforced.

One example of Congress occupying the field is the Federal Aviation Administration's regulation of air traffic control. Because the federal government's interest in air safety is so important, state laws regulating air traffic control would be preempted by federal law.

Another example is the Federal Communications Commission's regulation of interstate telecommunications. Because the federal government's interest in regulating communication across state lines is so dominant, state laws regulating telecommunications would be preempted by federal law.

These examples illustrate how Congress can use its authority to preempt state laws in certain areas where federal regulation is necessary for the greater good. By occupying the field, Congress ensures that there is a uniform set of rules and regulations across the country, rather than a patchwork of conflicting state laws.