A simple airfoil has a flat bottom and a curved top surface. This means the air has to travel a little further over the top of the wing to meet up with the air that's going across the bottom. To do this the air over the top has to speed up (it's got further to go) and by speeding up its pressure drops. Now there's less air pressure on the top of the wing than on the bottom and this is what the wing feels as lift. But a symetrical wing section has the same curve top and bottom which means that if it's facing directly into the wind then it can't make any lift because the air is moving at the same speed over the top as it is over the bottom surface. To get that type of airfoil to provide lift it has to be angled up very slightly. This is called the angle of attack. It only needs a very slight angle of attack for a symetrical wing section to provide enough lift to fly level, maybe 1 degree or even less depending on what the wing loading is. The beauty of a symetrical airfoil is that the model is just as happy flying inverted but will still need that slight angle of attack by giving a tiny bit of down elevator.

Edit...it seems the previous post was sent while I was typing .