What is Angle of Attack?

Angle of Attack is the angle between the chord line and the relative airflow (sometimes called relative wind). All wings will stall when they reach the critical angle of attack which is between 16-18 degrees.

Angle of attack is greatest at low air speeds in relation to your plane. 40KTs could be slow in a Cessna 152 while 80KTs might be slow in a light twin engine plane such as a Piper Seneca. Even though the speeds are different in each plane it is possible that at that speed they would both be flying at the same (high) angle of attack.