Hi Sasha, That means Tv channels show violence to attract the audience as they know whenever scenes of violence and sex are on people watch them!

My teenage boys are more likely to buy a computer game, movie or comic book if there is violence involved rather than cuddling or cute puppies. Violence sells can be extrapolated to: The inclusion of violence in products; especially toys, games and media are more likely to achieve higher sales than those without.

It is informal language. "Sells" means "tries to sell and succeeds." Suppose a car company runs a campaign saying "Our car is safe," and they don't sell any more cars. Now suppose they run a campaign saying "Our car is powerful," and their sales increase. The car company might say "Safety doesn't sell; power sells." "Violence sells" means that game companies find that they sell more copies of their violent games than their other games. "Violence sells" isn't a common pairing of words; "sex sells" is more common.

Also, a side note: the word "sell" can have a variety of meanings, both formal and informal. You can "sell" an idea; for example, "The President tried to sell his policy proposal to the public in a speech." You can also "sell" a song or a performance, in the sense of really engaging the audience: "In order to really sell a song, it isn't enough to sing in tune; you need to have stage presence as well." It also can mean a swindle, cheating, deception. (Informal language) "You joined a multilevel marketing scam. I hate to break it to you, but you got sold."