Yes, I think it is saying Low AP. what does that mean?
AP is action points. The number of shots you can take in VATS is limited to how much AP you have to spend. Different weapons use different amounts of AP per shot in VATS. Usually the stronger the weapon the higher the AP cost.
Once you use up your AP you must let it recharge while outside of VATS.
Your AP bar is the bar above the numbers telling you how much ammo your equipped weapon has.