A more thorough explanation:

Plenary power

Plenary power refers to complete and unrestricted power over a specific area. This term is often used to describe the Commerce Power of Congress. According to the Commerce Clause (Article I, Section 8, Clause 3) of the US Constitution, Congress has full power over interstate commerce. This means that states cannot pass laws that affect interstate commerce without the permission of Congress.

One example of plenary power is the power of the federal government to regulate immigration. The US Constitution grants Congress the power to establish a uniform rule of naturalization, which means that Congress has complete control over who can become a citizen of the United States and under what conditions.

Another example of plenary power is the power of the federal government to regulate foreign trade. The US Constitution grants Congress the power to regulate commerce with foreign nations, which means that Congress has complete control over the import and export of goods and services between the United States and other countries.

Plenary power means that the government has complete and unrestricted control over a specific area. In the case of the Commerce Power of Congress, this means that Congress has the power to regulate interstate commerce without interference from the states. The examples of immigration and foreign trade illustrate how plenary power works in practice. In both cases, the federal government has complete control over these areas and can make decisions without interference from the states.