It's your ship's cargo door, a purely aesthetic object. The large room on the lower floor of the ship was designed as a hangar, and that cargo door would realistically allow smaller vessels to enter and leave your ship. Back when I first built the ships, there was no support for zero-G environments, so while I did consider making that door functional, I ultimately decided against it as it just caused too many issues. With the new vanilla space update, it may or may not be possible to add a zero-G environment outside the ship now. Unfortunately, making changes to the ships at this point could cause issues with existing characters, so even if I can make the door functional now, I'm not sure if it'll be worth the trouble...

So for now ill fill the hanger with lava. Hopefully they allow mechs to go in doors like that soon.