Subdominant
The subdominant is the fourth degree of a scale, and the chord built on it (IV), which moves away from the tonic without pulling back towards it.
What is the subdominant in C major?
In C major the subdominant note is F and the subdominant chord is IV: F–A–C. In A minor it is D, and the chord is minor, iv: D–F–A.
The name describes a mirror. The dominant, G, sits a fifth above the tonic C. The subdominant, F, sits a fifth below it. It is also, more plainly, the degree just under the dominant.
The ii chord does the same job as IV and often replaces it. In C major, ii is D–F–A, which shares two notes with F–A–C.
How does the subdominant sound?
Like departure. IV has left home, but nothing in it is straining to get back. The dominant contains the leading tone, a note one semitone below the tonic that leans into it. The subdominant has no such note: every note of F–A–C sits comfortably in the key.
That is why the order of the three main chords matters. I–IV–V–I is home, away, tension, home. Play V–IV–I instead and it sounds like a step back, because the tension arrived early and then relaxed.
Is IV–I just a weaker V–I?
No. IV–I is a cadence in its own right, the plagal cadence, sometimes called the “amen cadence” because it is the shape of the sung a-men at the end of many hymns. It settles where V–I resolves. The difference is the missing leading tone, not a lack of strength, and plenty of songs choose it on purpose for a softer ending.
Try it: The Four Cadences plays short progressions and asks how each one ended, plagal included. What the Degrees Do drills the three functions on their own.