Here's how I actually did explain it to a co-worker..
"That cool shot on my workstation desktop that I took through my telescope? I went there in the game last week. A few weeks before I dropped in on the black hole at the center of the galaxy. The entire galaxy is in there. At 1:1 scale. You can do whatever you want. You start with a few credits and a secondhand beater of a ship and you're on your own. The game gives you nothing, you trade, smuggle, get into trouble with the cops, get out of populated zones while the heat is on so you can mine in peace and occasionally you meet other players doing the same things. Some will help you, some will rob you and some will just shoot you for laughs. But remember that 1:1 scale thing. Unless there's some kind of "attractor" like a known event or a hot trade route expect to meet other people VERY rarely. 99% of the other ships you meet are AI piloted."
He expressed an interest and I told him more, but was sure to include this warning...
"Don't expect the game to give you anything, don't expect a 'storyline' other than one you make for yourself. If you want something you'll have to take the time to earn the credits and find the pieces you need to get it and you get no help with that. Being one guy trying to make it alone in a big galaxy can get to feel like hard work sometimes and if you mess up you can lose it all and be right back where you started. If you're the kind of gamer that absolutely has to grind out a max level character that's got so much legendary gear that he can one-shot the Gods and you can hear him clanking when he walks from half the map away, you will find a LOT of grind in Elite and it WILL get frustrating. If you try playing it like that and need to replace your keyboard because you broke it from smashing your head repeatedly into it, don't come and blame me because you were warned. However, if you find other kinds of fun in games there's likely something in ED you can find to do that you'll like. Just don't expect to like it all. I'm totally hooked on the game and I don't. I just don't do those things, because 'no storyline' means I don't have to."
This was just before Horizons dropped, and he picked it up when that happened. He's still playing and (if this is even possible) he's now even more hooked on it than I am.