Simplify the following text for a Grade 7 native English-speaking person.
Original: A chord progression ( or harmonic progression ) is a series of musical chords , or chord changes that `` aims for a definite goal '' of establishing ( or contradicting ) a tonality founded on a key , root or tonic chord . Chords and chord theory are generally known as harmony .
Simplified: A chord progression is a part of music , made up several of chords played one after another .