Refer to figure.
The boundary layer is the layer of fluid (in our case this fluid is the atmosphere) in the immediate surface of a bounding surface (the airfoil) where the effects of viscosity are significant.There are two types of boundary layer; the laminar and the turbulent boundary layer.

In the laminar boundary layer, the flow moves parallel to the surface over which it flows.In the laminar boundary layer, the air flows very smoothly.The velocity increases with distance from the airfoil and there is no movement of air perpendicular to the surface of the airfoil.Laminar flow creates less friction drag than the turbulent flow, but is less stable.

At a distance from the leading edge of the airfoil, the air transitions from a laminar flow to a turbulent flow.This air does not flow smoothly, it contains swirls.There is flow in parallel and perpendicular direction.This air creates more drag than the laminar air flow, but breaks down less easily and is therefore more stable.