The most common rotary engine in use today is the Wankel engine. There are three apex seals in a Wankel engine. They are attached to the three points where the rotor is closest to the housing. A central rotor with three points rotates inside a housing. Two spark plugs fire the fuel. The input of fuel and outtake of exhaust occur as the rotor moves. The spark plugs fire once every time the rotor moves around. Each point on the rotor is called an apex, so the three seals are apex seals.
Seal the Gap
Each apex seal has to close the gap between the rotor and the housing. They push the fuel into place, allow an ignition chamber to form for a split second every time the rotor passes the spark plugs, and then push the exhaust out. This happens very fast. The seals must keep the fuel and exhaust gases in place very reliably at high speed and hot temperatures.
When a rotary engine fails, it is often because an apex seal has broken. Made of steel or ceramics, the seals have to work or the engine will not work well. The rotors provide the power but the seals make the system work.