Maps, also known as cache files, are bundles of processed tags. They allow Halo to efficiently load the resources necessary for a level as opposed to individually loading and processing loose tag files.
Maps are often all you need to share a mod, though sometimes a mod also requires custom sound banks, locale bins, or shared maps. Maps and other needed resources can be published to the Steam workshop using Excession.
Shared maps and resource maps contain tag data common to multiple levels. When building maps, Tool will include all the tag data needed for a scenario unless it's already present in a resource map, in which case it's referenced instead. Creating custom resource maps is an advanced workflow that you don't generally need to do.
For technical details on Halo 1's map file format and resource maps, see its dedicated page.