You plug the device into your car (mine was under steering rack) and it will tell you if you have any faults and what they are. Also tracks fuel consumption and will tell you where your car is etc. Brilliant in my opinion as you can work out what exactly needs fixing on your car before going to a garage. Save the diagnostic fee and make sure you're not getting ripped off by checking ECP etc. Works with pretty much all modern cars.
Just bought mine at full price however am still very happy with it. Worked brilliantly. Just know that you have to connect through the app rather than with your phone's bluetooth system.
They also have a device for iPhone but its a little bit more. I believe it would come to £19 after the coupon code.
Also BUY 2 GET 1 FREE if you wanted to split the cost between friends/family and save even more.