Like Brando said,
for example if you have Xfinity internet and bought or are renting a "xfinity" modem/router from them,
Your router is actually making a public hotspot called "xfinitywifi" in addition to the home network you set up, and strangers or the kid next door can use it. Maybe someone next door to you is accessing that open network and watching TV all day.
Here's how to turn it off for xfinity:
https://www.xfinity.com/support/articles/disable-xfinity-wifi-home-hotspot