Master Bundles allow content (textures, meshes, audio, etc) for multiple game assets to be bundled at the same time, rather than into individual .unity3d files. (This is probably how it should have been in the first place, but anyway...) While slower to build, they have the benefit of updating many bundles at once, and are more RAM-efficient because any common files between the bundles are shared rather than duplicated.

When Unturned finds a MasterBundle.dat file it will assume all child folders are using this bundle with paths relative to the .dat file, unless Exclude_From_Master_Bundle is specified in the individual asset .dat file. For an example of this, check out the Greece/Bundles and Hawaii/Bundles folders.

Another effect of this change is that it will be easier to separate some maps from the game to free up space for more future maps. Specifically, I'm planning to move Hawaii + Germany and then Greece + Cyprus, to the workshop rather than in the main download in a future update. All Germany assets will continue to be bundled with the game, only the map itself will be moved. Hawaii and Greece assets will be available as a separate download on the workshop for maps which would like to add them as a dependency. Simply copy MasterBundleTool.cs and MasterBundleHelper.cs to your Assets/Editor folder.