In short it's a virtual storage for grids, whether it be a static grid, large or small one. You can store up to 20 grids per sector and once you are offline for 3 days all your grids are moved to hangar automatically to free server resources for active players.
In contrast to seamless plugin which is a client-side plugin, hangar plugin is running entirely on server and does not require any installation from players.
All hangar commands starts with !hangar but we recommend using simple !h to avoid misspelling it as hanger (it might but surprising but this is the most common issue with hangar).
!h list!h list command, then find your grid number and do !h load # where # is number from list. Grid is loaded in the same place where it was saved, take especially care with rovers as they will be put some distance above ground.!h save, it will be saved to hangar and removed from world.!h list if you are on ie. Abydos