Toilet tank cleaner is a solution deposited into the tank of the toilet, as opposed to the bowl. It is used to remove the build up of rust, calcium and hard water that forms over time inside the tank. This process also helps a toilet tank last longer, and look and smell cleaner.
Some people flush toilet tank tablets in the bowl of their toilet, which can actually damage the rubber gaskets inside the tank. The best way to clean the tank of a toilet is to pour bleach into the tank on a weekly basis, wait 10 minutes, and flush the toilet 3 times.