Diatonic (Greek: διατονική) and chromatic (Greek: χρωματική) are terms in music theory that are most often used to characterise scales, and are also applied to musical instruments, intervals, chords, notes, musical styles and kinds of harmony.
Although many people aren’t aware of it, a typical piece of music actually follows a certain set of “rules,” when it comes to which chords are used. That is, not just any chords can be used- only certain chords are used. Some chords belong, and others don’t. As long as you know which chords “belong,” then you can create music that has a certain “unified” feel to it. That’s a powerful thing! This knowledge is like a “secret code” that helps composers to compose, improvisers to improvise, and performers to perform.
A diatonic chord is a chord which uses only notes from the “current scale”. So, say you’re writing a song in the key of Bb major, using notes from the Bb major scale. Then the “current scale” is the Bb major scale, and diatonic chords are any chords which use only notes from the Bb major scale.