From TM Wiki

A media pack is an compressed archive file, usually in the Zip format, which contains a set of custom content media files (music, signs, car skins etc.) built for a specific track.

The pack includes the same directory structure like the TrackMania game it is made for, relative to the main directory of the game and depending on the media files to be included. This means you just have to put the archive file into the TrackMania directory, unpack it and all files will be located in the right place.

The advantage of a media pack is that you don't have to wait for all custom content to be downloaded when you start playing the track, especially useful if the track in question uses many media files, not just one or two.

Mods are usually not included in a media pack since those can get huge.