A media player is just a piece of software that runs on your computer and lets you watch TV or listen to the radio over the internet. Your computer probably has one already, but if not they are usually free to download.
A wide range of TV and radio programmes are available online. Sometimes you can watch them straight away by clicking a link on the programme's website and sometimes you can download them to enjoy later.
Media players usually let you pause, fast forward and rewind programmes as well as letting you watch home videos from your camcorder or play CD's and DVD's on your computer.