the angle the copter is tilted side to side. Think… leaning left and right. Rolling causes the copter to move sideways.
the angle the copter is tilted front to back. Think… leaning forwards and backwords. Pitching causes the copter to move forward and backwords.
the angle of rotation the copter is experiencing while keeping the copter level to the ground. Think… sitting in a chair and spinning round and round without rolling or pitching.
The next post will describe in more detail, how to utilize roll, pitch and yaw at the same time in order to steer the copter.