A A Minor chord contains the notes A, E, C. It is built from the minor formula 1 - ♭3 - 5, with A as the root.
What is A minor the relative minor of?
A minor is the relative minor of C major — the two share exactly the same notes and key signature, with no sharps or flats. That's why Am and C sound so natural together and why swapping between them is such a common beginner exercise.