Dehydration is removal of water.
In alcohols dehydration is α-β elimination or 1,2 elimination, it means the hydroxyl group will be removed from α-carbon while the hydrogen will be removed from near by carbon.
In case of neopentyl alcohol there is no β hydrogen present on the β carbon [as shown in figure].
The only possible way for it to undergo dehydration is by rearrangement.
The process or mechanism can be understood as:
so the chief product is 2-methylbut-2-ene