In music, the subdominant is the technical name for the fourth tonal degree of the diatonic scale. It is so called because it is the same distance “below” the tonic as the dominant is above the tonic – in other words, the tonic is the dominant of the subdominant. It also happens to be the note immediately “below” the dominant. It is sung as fa in solfege. In the C major scale (white keys on a piano, starting on C), the subdominant is the note F; and the subdominant chord uses the notes F, A, and C. In music theory, Roman numerals are used to symbolize the subdominant chord as ‘IV’ if it is within the major mode (because it is a major triad, for example F-A-C in C major) or ‘iv’ if it is within the minor mode (because it is a minor triad, for example F-A♭-C in C minor).

https://en.wikipedia.org/wiki/Subdominant