A major chord is a basic triad made of the root, a major third and a perfect fifth. On C, that means C, E and G.
The basic idea
It readily creates a bright, open, stable impression and forms the foundation of chord progressions.
What to check when using it
In the piano roll, place a note four semitones above the root and another three semitones above that to see how it is built.
A minor chord has a different third, which changes the character of the sound.
