I'm not sure...check Sheldon Brown or google images ("chain tool"). The chain tool pushes the little peg out of the chain link (but not all the way) so that the chain will come apart. Then it pushes it back in once you've fixed the chain. You get dirty and a bit greasy.

If your chain breaks, you can shorten it by one link and it will get you home (unless you ride a single-speed, in which case sometimes the chain becomes too short).

Once you master fixing flats, I suggest practicing using the chain tool on an old chain.

--Julie