Murray,
It's a standard textbook rule of music theory, and is quite well known by jazz guitarists, bassists, and pianists, less well known by folkies.

Simply put, all seven chords in the diatonic scale (the white keys on the piano) can be divided up into four categories or functions. These are:

• Tonic
• Subdominant
• Dominant
• Diminished

Using the key of C for example chords that fall under the Tonic function are:

• C, Em, and Am. These are Stable, at rest

Chords that fall under the Subdominant function are:

• Dm, and F. These are unstable and moving.

Chords that fall under the Dominant function are:

• G7 This one is very unstable and moving.

Chords that fall under the diminished function are:

• Bdim, and Ddim. This chord is also very unstable and moving, resolving to the C chord.

Probably clear as mud without my little hand out sheet that I usually use to teach this subject. But I hope this helped some.
Jim