What is boundary layer, exactly?
Airflow (or any fluid flow) will be zero at the surface of the object it's passing over. Imagine the airflow as layers (or "streamlines"), and remember the surface layer velocity is zero. As you move away from the surface, each layer will be going a little faster than the one beneath it until you finally reach "free stream" velocity. All of these low-energy (lower velocity) layers of air between the aircraft surface and the free-stream air make up the boundary layer.

Here is an illustration from NASA:

(NASA)