@ Lieven
the 5th of the 5th is a great thing.

@Lord_Gigabyte
If you play in the key of C major, the I chord is C and the V chord is G (5th chord in a sequence of 7 "diatonic" chords).
If you're playing in the key of C and you play a G chords, you have a strong feeling it needs to "resolve" to the I chord-> to C. It brings peace to the galaxy.
But, you can spice up your progression by taking the V chord of that G in your progression and "resolve" to it. This is what Close2U meant with "fifth of the fifth"; you can take the V chord of the V chord in the progression.
Because doing the V-I move really sounds like the "right thing to do" , it will work almost every time.
If you're not into diatonic chords, tihs might sound a bit complex but this is one of the many tools to turn your regular 4-chord popsong in something more interesting