That's a ferrite magnet. Just like the clamp on ones some of the R/C compaines try to sell you for $8 except it doesn't have the plastic casing and doesn't clamp. Oh, and it only cost about $1 at a local electronics store.
I also have one on the negative battery wire which you can't see. There supposed to reduce radio noise and my home track is kinda brutal sometimes, especially when we run 19 turn so anything that might help is a plus.