Think of all the people you see in the background at the diner in Seinfeld, Cheers, or even the Stormtroopers in Star Wars.
They say nothing, or pantomime gibberish to each other to make it look like they’re having a conversation.
Famous Extra Fact: James Michael Tyler was a ‘background’ actor in NBC‘s Friend’s café, Central Perk, when one day in the second season he was given a line, and suddenly he was the character Gunther on the show... for ten seasons!
The Assistant Director usually corrals and directs the background actors.
If you're interested in becoming an extra, there are many websites (try this one!) and resources out there. Remember that this is a profession, and the days can often be long, and in the elements. It's not for tourists. And also note that asking for autographs is extremely unprofessional and might get you tossed off the set.