The answer is quite simple.
If you have a TV without a HD tuner and you get your signal from an antenna, then on February 18, 2009, your TV will stop working unless you get a special "converter box". The converter box converts HD signals to an analog signal that your TV is capable of viewing.
Another thing for folks relying on antenna based TV reception to be concerned about is that the digital HD signals may not be as strong as the analog signal they are getting today. So, if you live on the fringe of reception, you may not be able to get the weaker signal.
If you have cable or satellite TV service, then you have nothing to worry about. Your TV signal provider will continue to provide a service that works with your current setup.