It replaces part of your normal video playback devices with a more reliable and universal file demultiplexer (you may be able to play more files or some more reliably). Once you install it it should automatically register itself in to any of the players you have (with rare exceptions). You can always uninstall it.
If that doesn't work, try getting ffdshow (replaces later part of video playback system). ffdshow will also automatically register itself. So just play your files like you normally do. You may have to go in the ffdshow config and enable the codecs though.