A mob is a big, rowdy group of people. You might avoid shopping at the mall right before Christmas because you can't stand the mob of shoppers.
A mob might gather around the piñata at a big birthday party or cluster in front of a music venue when tickets go on sale for a hugely popular concert. When people join one of these groups, you can also say that they mob, and "the mob" also sometimes refers to organized crime. You might describe the hundreds of people thronging the county fair on Friday night as a "mob scene." The Latin root, mobile vulgus, means "fickle common people."