Diatonic harmony. That's a fancy name for chords in the same key that use only the 7 major notes in that key. Don't you love that technical jargon.
For example, the major notes in the key of C would be C-D-E-F-G-A-B. A chord progression in the key of C would be C-Am-F-G7. In Nashville numerical notation that would be 1-6-4-5, or I-vi-IV-V.