A Toilet Rim Block is a detergent block used in flush toilets which slowly dissolves in water. They are usually placed in a small holder (such as a Rim Cage) that is placed over the rim of a toilet and hangs down into the bowl. The water passes through the holder coming into contact with the block as the toilet gets flushed. These may also contain a coloring, which shows up in the water – typically blue.
Besides cleaning and giving fresh scent to the toilet, these blocks are also used to help prevent the build up of limescale in the bowl.