Extras are the people who appear in the background of virtually every TV and film drama. Look at Eastenders. The stallholders and shoppers and the customers in the Queen Vic who never speak scripted lines are all extras. Some are regulars but most are booked for a day or a few days for shooting a particular scene. They can be called for most it is a part time job done in addition to a full time job in entertainment or elsewhere.
Extras require some acting skill but are not regarded as actors, who get named in the credits at the end of the show and have speaking roles. There are higher grades of extra who will portray a particular character or interact with a principal or speak a few inconsequential words. These get paid more and are either booked in advance or, sometimes, picked on set.
In so far as screen drama portrays life there is a need for all types of people: characters and neutral anonymous faces, old and young, smart and scruffy. In fact, everyone you might ever meet has a place as an extra.
You do need to be able to drop everything and take work when it is offered (this is not usually a full time occupation). You must get yourself to a studio or location with less than 12 hours to plan (locations can be remote from public transport) and be ready to get up earlier to make whatever time you are asked to be on set.