I hadn't realized you weren't familiar with it. That could explain some of the confusion on this thread, though!

Etiquette involves a set of guidelines that make social interactions easier, so you're less likely to inadvertently offend someone. You might want to look through books or websites by Emily Post or Miss Manners if you want to learn more about it. Not everyone follows it these days but it's certainly handy for avoiding uncomfortable situations. By "mainstream etiquette", I mean the etiquette guidelines that are observed by the majority of people in the US (and are found in etiquette books), since obviously there are regional variations in what is considered acceptable, and some social circles have different customs as well.