When you download a tv show or movie you have to be able to watch it on your computer or video player. To watch it on your computer you will need to install a codec pack.
The word codec means to compress and decompress.
Codecs represent how a video is compressed from its original source down to relatively small file and then uncompressed by your media player at home so you can watch the movies.
Most codecs lose video information during the process so the trade off between file size and quality is important.
The most popular codecs at the moment for downloaded movies and tv shows is the Divx and XVID codecx.
The important thing to realise is that you need to have installed on your computer the Divx or Xvid codec that was used to compress the video or you will not be able to uncompress it and watch it after you have downloaded it.
Whilst you can get a hold of individual codecs it is often far easier to install one of the codec packs. These are collections to ensure you have all the codecs you need to play just about every movie and tv show when you download.