 Here is a molecule. It has two carbons, and therefore we'd call it eth-something. It has a single bond between the two carbons, so therefore we would call it ethane. Now, we would leave it at that, except for the fact that it has this atom where a hydrogen should be. This green one is a chlorine atom. If we add a second chlorine, there's now two places that we could add that. We could add the chlorine to the same carbon. Now we don't have a chloro, now we have a dichloro.