Before there was cable or satellite there was Over The Air (OTA) broadcast television. You simply put up an antenna, have your TV scan for channels and enjoy the major networks like ABC, CBS, NBC, etc.
Go to www.antennaweb.org and it will show you where the TV towers are around your home and what channels you can get with an indoor vs outdoor antenna.
If you want to do this with a PC instead of a HDTV - you must purchase a TV Tuner card, install it and run the setup software.
BROADCAST vs CABLE
There is only enough frequency in any town for a few dozen channels. So they created "CABLE" channels and now dedicated channels of shows (ESPN, CNN, Discovery, FX, TCM, etc) can be sent to peoples homes. But these "Cable" channels are never broadcast. You must have a Cable or Satellite service to see these.
Satellite box's are 'smart' these days. The signals are encrypted, the encryption changes every few days and only box's tied to active accounts get the updated decoding keys.
Nothing you can buy will let you get these signals for free.
CONCLUSION:
OTH broadcast channels are free with an antenna. Everything else requires a cable or sat subscription + gear.