Not a stupid question at all. To put it simply, AoA (angle-of-attack) determines how much drag force (force that opposes motion) the airplane will produce. In general, the higher the AoA the higher the drag. Every airplane has it's optimum AoA where it has the lowest lift-to-drag ratio. In airliners this number is usually around 4 degrees and this is what determines the most optimal cruising speed.
Think of AoA as an abstraction of the airplane's performance. We usually use speeds to do this, 'cause as humans this is something more tangible for us, but if you think about it AoA is the true performance indicator. What I mean by this is think of approach/landing speeds for the same airplane at different weights: you would need to know what speed is perfect for each weight. If you use an AoA, and you know what is the best value for it to use on landing, then you don't even need to care about your airspeed. As long as you have AoA value correct, what ever speed you are going at is the correct one. Same goes for stalling, cruising, etc.