It's a device called a Gurney Flap, invented by race driver Dan Gurney in the late 60's early 70's and is one of the rare things that have transferred from motor racing up to aviation, instead of the other way round.
I'm really tired right now and it's probably not the technically correct description but I believe Gurney Flap 'fools' the airflow into making the aerofoil longer than what it really is, but not adding a lot of drag.
(probably wrong, it's all I've got sorry)
The other motor racing name for them is a wickerbill.