Boundary layers, explained with a river bank
The slow layer against a surface, and why it governs everything.
A boundary layer is the thin region of air right next to a surface that has been slowed by friction against that surface, and almost every important thing that happens to airflow over a car, from grip on a floor to separation off a wing, is decided inside that thin, slow layer.
Standing at the edge of a stream
Air does not slide past a solid surface at full speed. Friction drags the closest layer of air almost to a standstill relative to the surface, each layer further out is dragged along a little by the one beneath it, and so the speed climbs gradually from nearly zero at the surface to the full, undisturbed speed a short distance away. That graduated region is the boundary layer. On a car-sized object it is often only a few millimetres thick, yet whether the air near a surface stays attached and well behaved or breaks away into a disorderly wake is settled inside it.
Wading at the edge of a flowing stream shows the same gradient through the legs. With feet almost touching the bank the water is barely moving, held back by friction against the bank, while a short step further out it drags hard and steadily against the shins at close to the river's full speed. The bank plays the role of the car's surface, the fixed boundary against which the flow's speed must fall to zero however fast the main current runs. Walk along the bank past a smooth stretch and then a rough, weedy patch, and the slow zone near the edge widens faster over the weeds, just as a boundary layer thickens faster over a rough surface than a polished one.
Why a curve far back behaves worse than one at the front
How thick the slow layer grows, and how well it clings before separating, depends on how far the air has already travelled across the surface and how sharply the surface asks it to change direction. The layer can be smooth and orderly near the front of a surface and churned and chaotic further along, and those two states carry different amounts of energy for following a curve. A boundary layer that starts thin near the front of a car can arrive at a curved section further back thick and close to giving up, with nothing about the free-stream speed having changed. The same curved panel can therefore behave quite differently depending on how much flat bodywork sits ahead of it, meeting either a tired layer or a fresh one.
Much practical aerodynamic work is consequently about keeping this layer thin, energetic and attached for as long as possible. Small features that seem fiddly for their size, a trip strip, a vortex generator, or a change in surface finish, can have an outsized effect, because they act directly on the governing layer and a well-placed intervention there changes the whole flow downstream. It is also why development tends to proceed surface by surface, since a fix for separation in one place can only be judged once the condition of the boundary layer arriving there, shaped by everything upstream, is understood.