What does the ballast resistor do?

It didn't smoke today. The piece is about 2-3 inches long and has a square metal cover over it. There are two wires connecting to it. One from each end. At the end of each wire there is a fitting that has a hole in the middle and a nut goes to it. Similiar to a mini battery cable. The end of one of them is white, kinda like it was burned. The other is plain silver.

They have a black protective sleeve over them, but I can see one of the wires is black with a white stripe.

Hope this helps.