Mat etiquette is a set of rules and guidelines for behaviour whilst training. They are designed to make the dojo a safe and friendly place for everyone to learn.
Most of the rules are common sense or simply polite. For instance, if you leave the mat for any reason you should let the instructor know. Jewelry should be removed in case it catches on anything while you are training. Since in Ju Jitsu training is done is bare feet and often with partners, cleanliness and personal hygiene is also important.