The mouthpiece, as the name says, is held in your mouth and you breathe through it when diving. You will find one on your regulator and octopus, as well as on your inflator.
Mouthpiece on the regulator
If you have already snorkelled once, then you will know this little fixture at the front of your snorkel. The mouthpiece of your regulator is similarly built on. So that it does not fall out of your mouth, it has so-called “bite tabs”, so little wings. With the lips, you firmly enclose it so that no water can get into your mouth.
After every dive, the entire equipment is usually washed in a fresh water bath with some soap. Should you still worry regarding the cleanliness of your mouthpiece, in the meantime there is the possibility to take your own with you on your trip and to exchange it for the one given to you on site.
Before every dive, check whether your mouthpiece is damaged. If a “wing” tears off during the dive, it will be difficult to keep the regulator in the mouth. In this case you could only use your alternative air supply. This one though would not be available anymore for its actual purpose in case of emergency.
Find the best Dive Professionals, Dive Centers and Liveaboards on rateyourdive.com