Boundary Layer Theory explains how fluid flows over a surface, such as an airplane wing or a ship hull. When a fluid, like air or water, moves past a solid object, it slows down near the surface due to friction. This slow-moving layer is called the boundary layer, and it affects the overall flow and drag on the object. The thickness of the boundary layer varies with the flow speed and surface shape. Understanding this theory helps engineers design more efficient vehicles and structures by minimizing drag and improving performance. Key applications include aerodynamics and hydrodynamics.