A wax ring is a donut-shaped seal, made of pure petroleum wax, placed between a toilet's base and its floor flange. It compresses when the toilet is set in place, creating a watertight, airtight seal that stops leaks and blocks sewer gas from entering the bathroom. It is designed for single use.
A wax ring sits between the base of a toilet and the closet flange. The flange is the pipe fitting anchored to the floor that connects the toilet to the building's drain line. Without a working seal there, every flush would leak wastewater and sewer gas out around the base of the toilet instead of down the drain.
The ring itself is a donut of soft, moldable petroleum wax. When the toilet is set onto the flange during installation, the weight of the fixture presses the wax flat. That pressure forces the wax into any small gaps or uneven spots between the flange and the toilet's outlet. This is what makes the seal work: the wax molds itself to whatever rough surface it meets. A rigid gasket cannot always do that as well on an older or slightly uneven floor. The result is a connection that is both watertight and airtight. That matters for two reasons. It keeps wastewater from seeping onto the bathroom floor with every flush, and it keeps sewer gas from escaping into the room between flushes.
Plumbing code does not name wax by itself as a requirement. What it requires is that the connection between a floor-mounted fixture and its flange use an approved gasket, washer, or setting compound, sealed properly. Wax has been the default choice for decades. It is cheap, reliable, and easy to install without special tools. But the code language also allows foam or rubber gasket alternatives. Some installers prefer those in spots a wax ring handles poorly, such as a flange set unusually low or high compared to the finished floor.
A wax ring is single-use. Lifting a toilet off an installed ring destroys the seal, even if the wax still looks fine. Any time a toilet comes off for repair or replacement, it needs a fresh ring going back down. Picture a toilet that has started to rock slightly and smells faintly of sewer gas near its base: that combination usually means the wax ring below it has failed and needs to be replaced before the leak spreads to the subfloor.