According to Microsoft:
A codec compresses or decompresses media files such as songs or videos. Windows Media Player and other apps use codecs to play and create media files.
A codec can consist of two parts: an encoder that compresses the media file (encoding), and a decoder that decompresses the file (decoding). Some codecs include both parts, and other codecs only include one of them.
So it has a huge importance in order to play the music or the video smoothly. And if it is broken then? The music or video file can not be played. Codec Tweak Tool is a very handy tool which scans the system and identifies the broken codecs which are not allowing you to play the music or the video file.
The application can be installed independently or as part of the K-Lite Codec Pack .