A major seventh chord is a major triad with an added major seventh, the note one semitone below the octave. Cmaj7 is spelled C–E–G–B; Fmaj7 is F–A–C–E. Chord charts write it as Cmaj7, CM7 or with a triangle, CΔ7. The close distance between the seventh (B) and the root an octave higher (C) creates a mild, pleasant friction that sounds airy rather than tense.
In jazz the major seventh often replaces the plain major triad as the tonic, the chord a tune comes home to, for instance at the end of a ii–V–I in C (Dm7–G7–Cmaj7). Adding a ninth gives Cmaj9 (C–E–G–B–D), an even softer sound. In jazz lo-fi a loop that simply rocks between two major seventh chords, such as Cmaj7 and Fmaj7, can carry a whole track and gives that hazy, daydreaming quality listeners associate with the style.