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