Another example.
You have a guy hiding in a bush... obscuring terrain. You cannot see that enemy, but that doesn't stop you from guessing where that enemy can be, and launching magic in his direction. That bush doesn't stop your magic from working, it merely makes it harder. If they are not actively hiding from you, you can even determine their location within that bush automatically. So you know whereabouts to launch that pew pew magic. You make the attack roll, and the concealment makes it harder to hit them. It does not make it impossible however, you could still get lucky.
Some magics don't require attack rolls tho... a Healing Word ALWAYS works in its target. You don't have to know exactly where they are, you just know who you're targetting and whereabouts they probably are, and the power of magic does the rest for you. Magic Missile is the same... you just need to know what you're trying to damage with it, and whereabouts it is. It is not a precision tool... it is 'fire-and-forget' magic that does the rest for you.
