720 means the resolution of the screen is 1280 x 720 horizontal lines that are progressively scanned on to the screen. 720p's progressively scanned image means that it handles fast moving action much better than 1080i signals. It is a format that is ideal for sports, action movies and video games.
Sometimes you will also see the 720p resolution listed as 720p/50. The 50 means that 50 complete frames are shown per second.
Most TV broadcasts are either in 720p or 1080i formats.