A television is an electronic device that broadcasts entertaining programs you can watch and listen to. You and your parents might argue over how much television you're allowed to watch on school nights.
If you're North American, you probably call a television a TV, while in the UK it's common to use the nickname telly. The word television was first used for the very earliest versions of the TV, around 1900. The word comes from the Greek root tele, "far off," and the Latin visio, "sight." Other names suggested at the time for this brand new technology included telephote and televista.