Plain English Meaning
These are the three ways an airplane can be tilted or turned in flight. Pitch is the nose pointing up or down, roll is one wing low and the other high, and yaw is the nose swinging left or right. Knowing all three tells you exactly how the airplane is sitting in the air at any moment.
Full Definition
The orientation of the airplane about its three axes of rotation: pitch is the nose-up or nose-down angle about the lateral (wing-to-wing) axis, roll is the bank angle about the longitudinal (nose-to-tail) axis, and yaw is the nose-left or nose-right angle about the . Together they how the airplane is positioned in space to the and its .