Imagine a galaxy. A real one, with millions, upon millions of planets, solar systems, stars, etc. Now imagine a game, based around exploring a galaxy that large. Every planet is different, procedurally generated (so it's the same planet in that same part of the Galaxy for everyone), but in a way that every planet In the Galaxy is different, with different types of plants and life(if any), different environments, different compositions etc. You can get on ground level in first person and explore each individual planet, it's life, it's environment, for probably the first time ever, by anyone. The game itself is so big that everyone who ever plays it combined will probably never be able to see it all collectively. Each planet is vastly different from the last, with an unimaginably large area. You could spend days or weeks exploring a single planet, or years just trying to land on every planet, with no end in sight. That, is No Man's Sky.