What does "alpha floor" mean?
Alpha floor is a thrust function which provides TOGA thrust regardless of thrust lever position in Airbus. From Airbus Technical Digest FAST 20:


  Alpha Floor is a low speed protection (in normal law) which is purely an
  autothrust mode. When activated, it provides TOGA thrust. As the aircraft
  decelerates into the alpha protection range, the Alpha Floor is activated,
  even if the autothrust is disengaged. Activation is roughly proportional to
  the rate of deceleration. 
  
  ... at low speeds, if a rapid avoidance manoeuvre is required to avoid terrain, windshear or another aircraft, it is safe to rapidly pull the sidestick fully aft and/or bank and hold it there. The aircraft will pitch up
  to max Alpha, engage TOGA thrust and climb away. 


Note that this protection is only available under normal law and is lost under alternate law.