I'm reading Jazz Theory by Stuart Smith. In the chapter on functional harmony the author introduces the notion of strong and weak chords:
I, II, and IV are generally treated as strong chords. V, VI, and III are generally treated as weak (IIIm7 and VIm7 as representatives of the Tonic constitute a special case, which is discussed below.) As we shall see, VII has a context-dependent role in tonal music. Sometimes it is treated as a form of dominant, while at others it is used as part of a circle- of-fifths structure such as IV VII III. In either case it would be considered a weak chord in the sense used here.
Please could you explain what does it mean - strong and weak chords, and what qualifies a chord as strong or weak? Is it somehow connected with stable and unstable degrees they are made up from?