So are you wanting to basically have your phone's screen displayed really huge on your TV? Or are you wanting to get your TV play movies that are stored on your phone's SD card?
Tethering is the act of using your phone's internet connection with other devices. The most common form of tethering would be, say, if you were out in the middle of nowhere with your laptop and mobile phone, your laptop won't have an internet connection, but your phone does. So you could activate tethering on your mobile phone and your laptop would magically detect a new WiFi network in range (which is actually your mobile phone), and your laptop can then connect to it. Thus, you end up using the mobile data connection your phone has with your laptop, and your mobile phone becomes a bit like your broadband WiFi router at home.
Take a look at http://en.wikipedia.org/wiki/Tethering