On the original ZX80 it produced modulated RF to connect to your TV aerial socket as it wasn't common then to have any other inputs on TVs.
More modern TVs have a composite video input which makes the RF modulator redundant. The quality of the picture is much better via composite video.
In fact that is how I used to use my ZX80 back in the day with a wire clipped onto the input to the RF modulator connected to a 9 inch monitor my dad liberated from work!
