If you open your toilet tank lid and peer in you should see a rubber piece at the very bottom of the tank with a chain connecting it to the flush handle. This is what is known as the toilet flapper. It controls the flow of water from the tank into the toilet bowl. When the flush handle is depressed, the chain lifts the flapper allowing the water into the bowl and when the flush handle returns to neutral position the flapper is designed to fall back down creating a watertight seal.
A toilet flapper spends the vast majority of its lifetime submerged and as the rubber ages it has a tendency to become warped and brittle, no longer creating a strong seal. As a result, toilet flappers are one of the most common toilet parts that need to be replaced on a regular basis. The good news is that they are extremely cheap (cheaper than a toilet plunger for example), easily available, and very quick to replace. When purchasing a new toilet flapper try and choose a model that is made from as flexible rubber as possible as these tend to have a longer lifespan. Universal models are available if you’re not able to find a flapper with the same brand as the toilet.
Before you replace the toilet flapper, turn off the water to the toilet. You should find a valve located on the wall either behind or to the side of the toilet tank. Flush the tank to get rid of the water and then disconnect the chain from the flush handle. Most flappers have two small plastic arms that go either side of the overflow pipe in the middle of the tank. You should be able to pull these arms off the pipe and remove the flapper. If the arms are attached with clips you should normally be able to use the same clips on the replacement part. Once you’ve removed the old, faulty flapper just put the new one in its place. Then attach the chain to the flush handle making sure that there’s not too much slack in the chain.