Chords are collections of notes, usually played simultaneously, and usually emphasizing a root note either rhythmically or by placing it in the bass, or both. Chords are named for their root and chord type. Chords of the same chord type are related by transposition. More
Certain chords are more common than others because (among other reasons) their voice-leading possibilities—and thus the progressions they can participate in—are constrained by their underlying mathematical structure.
(Other reasons certain chords are more common include simplicity (low cardinality, three or four notes), consonance, etc. but these are related mathematically to voice leading potential and thus overdetermined, as explained at length by Tymoczko 2006 and Tymoczko 2011. See Orbifold & Voice Leading.)