Exhaust plugs are flexible pieces designed to protect the inside of the muffler from outside elements. In practice, the exhaust plug works well when washing the vehicle with pressurized water, so that chemicals and water do not penetrate inside. They are great for protecting against moisture during prolonged motorcycle parking in winter.
We can find smaller and larger plugs, two-stroke vehicles have smaller exhaust diameters and four-strokes have larger exhausts, hence the two types of plugs.