"Subdominant" also refers to a relationship of musical keys. For example, relative to the key of C major, the key of F major is the subdominant. Music which modulates (changes key) often modulates into the subdominant. Modulation into the subdominant key often creates a sense of musical relaxation; as opposed to modulation into dominant (fifth note of the scale), which increases tension.
In sonata form, the subdominant key plays a subordinate though still crucial role: typically, in the recapitulation, there is a section written in the subdominant key, occurring at the point corresponding to the location in the exposition where the music modulated into the dominant key. The use of the subdominant in this location often serves as a way of keeping the rest of recapitulation in the tonic, although this practice is extremely rare.