I walked outside yesterday and felt the wind blowing around me and I thought to myself "There's a boundary layer around me!" It was so exciting.
You see, a boundary layer is that layer of air right up close to some object that has fluid flowing over it. Suppose you were like me and you went and stood outside with the wind rushing past you. Because of friction, the air exactly next to your skin is not moving, it has a velocity of zero. And then as you get farther from your skin, the friction is less and the air begins to move slowly and so on, increasing in speed until it has reached the same speed as the rest of the flowing air around you. The section of air from right up exactly near skin to the point where is has increased in speed to the rest of the air is called the boundary layer because it is a layer all around you of air changing velocities. Now the reason you don't feel this is because it is so extremely small. But it becomes important when working with things like submarines or airplanes because the boundary layer is larger and affects the performance of the object more.
But they can be fun to learn about.