So, I didn't know what it was...but it turned out I kinda did and do already use it.
Very (very) simply, imagine taking the open (aka cowboy chords) version of C, A, G , E and D and playing them up the fretboard (using barre's). In my basic understanding, it's just inversions of the chord as you work through the shapes of C,A,G,E,D (and can be applied to whatever flavour of chord - major, minor, 7th's, etc...) . Though it get's a bit more involved than that when you start to link them together.
Useful for everyone, rhythm guitarists, lead players and even bassists!
All courtesy of a couple Paul Davids video - though many do talk about it, I found his overview quite practical!
If you are a complete beginner, some theory is real useful!
If intermediate (me!) and want a intro to CAGED system:
If you already been exposed to it and want to see where it can take you: