The yellowish film on the surface of the tongue contains bacteria that produce the unpleasant smell associated with bad breath. A tongue cleaner is a highly effective means of significantly reduce tongue based bacteria and should be used at least once a day. Tongue cleaners come in various shapes and sizes, but all work in the same way, by mechanically scrape this yellowish layer of biofilm from the surface of the tongue.
Some people find that using a tongue cleaner makes them gag. Rather than inserting the cleaner fully into the mouth it is better to stick out the tongue and start by just cleaning as much of the surface as you feel comfortable. Over time the gagging reflex will abate, allowing comprehensive cleaning.