To my gamer friends: It's a MMO(ish) RPG(ish), sci-fi game where you are a pilot of a spaceship in a giant universe most of which that wants to kill you.
To my non-gamer friends: It's like an unscripted (plot-less) version of Firefly. You fly a space ship and the goal is to keep flying, not crash in a big fireball, earn cash, upgrade the ship and get more skilled at flying.
Then let them ask questions, better to let them ask than try to force it on them.
For example my gamer friends asked about things like PvP, is it skill based, what would you compare it to etc. Most exciting thing and so-on.
My non-gamer friends asked what the point was and my answer was: "Having a point costs extra". Then being serious I said: "It's entirely up to you to make your own fun. Want to ignore the real world and forget all your worries just flying a space-ship for a few hours? Check. Want to explore a realistic simulation of our universe and explore the childhood fantasy of space-travel? Check. Want to meet up and race around a 9km long space station at several times the speed of sound. Check.
All what you make of it like most sandbox or roleplaying games."
Seemed to work ok tbh. Not many were interested enough to check it out but there were one or two that asked a few follow ups like "How accurate is the galaxy?" and "Are the physics realistic?" <- Lots of my friends did Science/Engineering degrees at Uni.
Edit: In response to some of the bits I read I felt it necessary to update. These were friends, people I'd known for 6 months plus and had mutual respect. Conversation naturally came to games/spare time and naturally moved on. The above were all passing and not a forced "hey I'm playing a cool game" or anything like that. I guess in those scenario's its harder as instead of discussing something it's more like you are trying to sell something and most peoples natural response is to oppose anything they feel is being pushed on them.