I just got the old starship, but I don't know what it does. I want to know before I lift off in case I die or something.
It seems (according to this site) that:
As you're wandering the map you will eventually come across a W. This is the landing spot of An Old Starship, which you ultimately have to fix up. Its hull needs work and its engines need upgrading; both require Alien Alloy. As you've probably discovered, Alien Alloy is quite rare…
… but, ultimately, worth it. A Dark Room's final challenge hinges upon worthy engines and a strong hull. Add a point to Engines and your ship's responsiveness will improve. Add a point to Hull and you'll be allowed another hit. Another hit in what sense, you ask? Simple enough…
… you're headed into space.
Once you take off you'll wind up in a fast-paced space sequence, soaring through what appears to be an asteroid belt. You need to dodge and weave through increasing numbers of space rocks and blast out into space. How much time you spend pumping points into Engines and Hull is up to you, but at least five points in each is recommended, if not more. It costs a lot to purchase Alien Alloy you don't find, so if you do come across any in the field, make bringing it back to your village a first priority.
Weave through the asteroids successfully and you'll complete the game… and loop back to the beginning. No rest for the weary.
So no, you won't die. You actually get something good out of it.