The sub-dominant note is the 4th note of the scale. This also has a strong resolution to the root, but not quite as strong as the dominant.
Together, the chords based on the root, the 4th and the 5th notes of the scale are referred to as the primary chords for that particular key. These chords are all major.
All other (minor) chords in that key are secondary chords (ie. chords based on the 2nd, 3rd, and 6th notes of the scale).