A TV tuner is a small component, either internal or external, that decodes a cable/satellite signal and forwards the video to a program on your computer that allows you to watch the broadcast.
For example, I have an Emuzed Angel USB TV Tuner from Dell (that I bought with my notebook). It is an external tuner. It has an input for a coax cable. It is connected to my notebook with a USB cable that it came with. When I have it all connected, I use Windows XP's built-in Media Center (only with the XP MCE edition) to watch TV.
The Media Center allows me to play DVDs, watch and record TV (like a Tivo), and has a nifty tv guide schedule.
You can also use this TV tuner to connect a console (ps2, xbox) to the laptop, because, in addition to the coax inputs, it also has component inputs (yellow, red, white).