It's a chip that goes in the boots to tell you how fast and far you've run etc.
I have it my boots also bought trainers that can take it too. Quite interesting to see the stats from it.
From getting them in my boots has made me switch from football to sprinting! Managed to clock 23mph in a game (allegedly same speed as Lennon, Young etc. also showed I run 8 miles a games.
It's more useful as a training aid to see intensity levels and distances covered.