It's a kind of map template which:
- is placed in its own zlevel (or collection of zlevels),
- should only get placed once per round,
- can be randomly picked to be placed automatically during round setup,
- normally has an overmap presence and shuttle landmarks so ships can fly to it.
It's a datum you can define in the code (/datum/map_template) which points at one or more .dmm map files, and allows the game (or admins) to spawn those map files when and where they please, whether that's into an existing zlevel (like decorating exoplanets with ruins), or on a totally fresh one (like an away site!).
tl;dr you make your .dmm files, then you write a new map template datum (/datum/map_template/ruin/away_site/insert_your_away_site_name_here).
BUT HEED MY RUMINATIONS
Away maps are expected to work whether you're on the main map, Runtime, or anything else you might want to load as the server's main map. That means your map mustn't use areas, turfs, objects, mobs or datums that are specific to any main map.
e.g. you can use /area/space, or /turf/simulated/wall, because neither are specific to a certain map. They live out in the main codebase, are always compiled in, and are available to all maps. But you can't use items that may only be compiled with a certain map, because they only exists when said map is compiled.
Except for, like, /area/space. That one's probably fine.
(Now that's just areas! Other stuff might be ok. Except very map-specific stuff as noted!)
Re-using areas is a shortcut, but it's bad for testability, and you can't guarantee that the area you re-use isn't loaded up and in active use on the server - in which case it'll hideously merge together with its brother in terms of stuff like air alarms, APCs, etc. Suddenly your derelict starship is randomly receiving power from another zlevel. Not good. Avoid. Make your own area types!
Not exactly a technical requirement, but it makes things easier in terms of collaboration. See how it's done with runtime-1.dmm, runtime-2.dmm, etc.
It's a pretty good place to put all your new types! #include other files here too. This file's going to be the hub of your map's wheel, to cack-hand a metaphor.
The game will read this to learn about your new shiny away sites, including what .dmm files it needs to load, how much it 'costs' to spawn (usually 1, increase for more performance-heavy away sites). Ask devs or read code if unsure..
The map loader works best when everything it's spawning does all its initialisation work in Initialize, instead of in New. Most likely your misbehaving thing is doing more initialisation than is healthy in New. It's fixable! Often not even that hard. Go talk to a dev.