Boundary layer theory explains how fluid flows over a surface, such as an airplane wing or a riverbed. When a fluid, like air or water, moves past a solid object, it slows down near the surface due to friction. This creates a thin layer, called the boundary layer, where the flow velocity changes from zero at the surface to the free stream velocity outside the layer. Understanding boundary layers is crucial in fields like aerodynamics and hydraulics. It helps engineers design more efficient vehicles and structures by minimizing drag and optimizing performance. The theory also plays a role in predicting turbulence and heat transfer in various applications.