Attitude means orientation in space.  For an airplane, a "nominal" attitude would be flying level, with the airplane's nose pointed in the velocity direction.  The actual attitude of the airplane is typically described using yaw, pitch and roll angles around this nominal attitude.  Specifically, if you lower the right wing, you have a roll angle.  If you rotate about a vertical axis, you have a yaw angle (this happens when flying in a strong crosswind).  If you point the nose down, you have a pitch angle.