The boundary layer forms the thin region near a surface where the flow is affected by frictional forces. How does this layer change along the flow direction?

The thickness of the layer increases as the flow progresses along the surface and the flow nature changes from laminar to turbulent until finally the fluid separates from the body.

Explanation:

The thickness of the boundary layer depends on it's distance from the leading edge of the body with thickness increasing along the flow. This can be attributed to the viscosity of the fluid that acts as a messenger that propagates the disturbance from the surface until the thickness where the velocity of fluid approaches the flow velocity of free stream.

Also the nature of the flow changes from laminar to turbulent which is attributed to the variation in the leading edge distance.