i bought some used springs and this came with it.what is it?

 

looks like a bumpstop to me.

 

and what does that do?

 

It's called a pinion snubber. It attaches above your differential. Since you're lowering the car, the differential will be closer to the bottom of the car and you have to change the snubber out to that one so it wont bang around.

 

oh ok that makes sense..thank you

 

no prob. You'll have to have the car jacked up with the rear axle hanging to be able to take it off. It's attached to the underside of the car by 2 or 3 bolts. Really easy to do.