A dynamic stall is a stalled condition different from a statically stalled condition, in that it is a transient, non-permanent condition, kind of hard to describe...
However, we are used to the fact that a given airfoil stalls at a given angle of attack. This (critical) angle of attack is the angle of attack that gives the highest coefficient of lift, ie, beyond this angle of attack, the CL decreases due to increased airflow separation on the upper side of the airfoil. This is measured in a static, steady-state, constant system.
However, the airflow separation on top of the wing is in reality dynamic and always changing, and during these changes you may see conditions, beyond the nominated critical angle of attack, where the airflow is less separated than in the above steady-state scenario. In practical terms, in some occasions, the stall will temporarily occur at a higher aoa than what is nominated in the paperwork. In practical use: read it, get it on the exam, forget about it.
Edit:
Dynamic stall - Wikipedia, the free encyclopedia