Rear Drag on Reel, what is it and how to setup?

I have just bought a nice reel and rod set off ebay - the reel has a Rear Drag nob on the back of it - i have never used one with this feature before. What is it and how do i set it up?

Cheers Fella's

doesnt matter - i worked it out - it's to adjust the spin on the spool when the line is being pulled out by a fish

thanks for. your help........not

[flaky-jakey@hotmail.co.uk]

its just for if you geta big fish,

and it weighs more than youre lines breaking strain,

line will come out,

so you dont get snapped