Imagine you would save all YouTube videos on your hard drive. You don't have enough space for that (and time to download anyway). So the next best thing is to just stream those videos and parts you actually watch.

And this is kind of how this game works; it will only deliver those parts and download in the background (which is called streaming) what you currently visit and need. Because you don't have enough space on your drive.

mbtrhcs wasn't saying that you specifically don't have a big enough hard drive, they're saying that MS Flight Simulator is simply too big of a game to completely store on a player's computer.

MS Flight Simulator has a fairly accurate 3D model of the entire earth. Like, the whole thing. So it's constantly downloading the parts that the player is currently in, and deleting the parts that they are not in.