A more thorough explanation:

A resident alien is a person who lives within the borders of a country but is not a citizen or subject of that country. In the United States, a resident alien is someone who was born outside the jurisdiction of the United States, who is subject to some foreign government, and who has not been naturalized under U.S. law.

Examples:

• An individual from Mexico who lives in the United States with a green card is a resident alien.
• A person from Canada who is studying in the United States with a student visa is a resident alien.

These examples illustrate that a resident alien is someone who is not a citizen of the country they are living in, but has legal permission to reside there.