A seventh chord takes an ordinary three-note triad (root, third and fifth) and stacks one more note on top, the seventh above the root. Changing the size of the third, fifth and seventh produces different colours. Spelled from C, the main types are the major seventh (C–E–G–B), the dominant seventh (C–E–G–B♭), the minor seventh (C–E♭–G–B♭), the half-diminished seventh (C–E♭–G♭–B♭) and the diminished seventh, whose top note is B♭♭, sounding the same as A.
In classical harmony the extra note usually adds tension that needs resolving. In jazz the seventh chord is the normal starting point, and plain triads can sound bare by comparison. Jazz lo-fi inherits this habit: the soft keyboard loops behind most tracks are almost always built from seventh chords or richer extended chords, which is a large part of their gentle, slightly unresolved mood.