Subsonic fluids < Mach 0.3 are considered essentially incompressible. That includes air and water. Now suppose you have a converging duct with air flowing through it, you have a controlled flow with a steady volume of air flowing through it at a steady rate (volume flow rate is constant). The duct opening is larger at the entrance (A1) than at the exit (A2), but since that volume flow rate is constant then the air must flow faster through the constricted section. So the exit velocity (V2) is higher than the entrance velocity (V1). This is referred to as Venturi flow.

V2 = (A1/A2) x V1

A wing may be flat on the bottom but raised on the top. So the air may flow straight and be unaffected across the bottom, with the same velocity as ambient air. However the bulge over the top of the wing acts like a venturi nozzle, constricting the cross-section of the flow of air against itself locally. Therefore the air flows faster over the top of the wing. The only difference is that flow over a wing is an external flow rather than an internal flow inside a duct. The effects on flow velocity are the same.

Picture if you will a flat plate, with air flowing over the top and bottom. There is no airfoil, no bulge at all. If you tilt it upwards at the leading edge (angle of attack) it will generate substantial lift. The main source of this lift is the dynamic pressure of the air pushing against the bottom of the plate. This creates a simple situation where the lift force is the sum of vectors between the airflow and the angle of attack.

The problem with a flat plate is that flow over the top will be unreliable and at a very small angle of attack the normally laminar airflow will separate to become turbulent (flow separation). An airplane where the wing is the sole source of lift and stability, flow separation means the wing will stall, uncontrollably losing its angle of attack and lifting ability, and possibly lose control of the airplane as well.

What an airfoil does is provide provide lift in the same way as a flat plate, but the smooth bulge over the top surface (airfoil) keeps the airflow laminar even at high angles of attack. In reality the lift is provided by the pressure on the bottom of the wing, and the normal airflow on the top. Lift from a wing is not provided by venturi flow, or increased airspeed.

A complication of venturi flow over wing's top surface can be seen on big jet liners that fly at high altitudes. The aircraft may be flying at Mach 0.7 - 0.9, but the venturi flow causes the airspeed over the top of the wing to become supersonic. As such a shock can develop on top of the wing. This shock can cause premature flow separation and stall the wing. Interesting visual effects are the condensation of water vapor at the shock boundary, similar to the following image: