It means a progression that lasts either 12 bars (or measures) or 8 bars (or measures). You've probably heard it a ton, a lot of classic blues rock is 12 bars. A good popular example is Cream's crossroads or Led Zeppelin's Moby Dick... or The Lemon Song... or Rock n Roll.

Edit: come to think of it. Almost all of Clapton's early stuff is 12 bar blues. Strange Brew... Crossroads... Hideaway... Steppin Out... Sunshine. They all use I-IV-V 12 bar blues too.
Last edited by al112987 at May 20, 2011,
12 bar blues is I I I I - IV IV I I - V IV I I

if you don't know chord notation like that its like c c c c - f f c c - g f c c

songs that use it that spring to mind instantly are johnny b good and spiderman
no sir away a papaya war is on