Codecs are technical procedures used to encode and decode video and audio signals. Through the use of codecs, video and audio files can be recorded and played by set standards.
Modern Windows systems come with a couple of codecs for various audio and video formats out of the box, e.g. MP3 and WMV. Some applications may however request procedures for which no suitable codec is available. In this case, you will have to manually install the required codecs.
There are various free codec packs available on the Internet. We recommend the "Full" edition of "K-Lite Codec Pack" that you can easily obtain through common search machines or software portals.