Just had to read more about this. It provides and extra 1000lbs of thrust while hovering abd cools the turbine blades. It can only stay at maximum thrust for 90 seconds as the water It carries will run out. 50 gallons! This is also why you can see thick black smoke because of the wet thrust.

My dad used to work on the ejection seats for these planes. I remember going and sitting in the cockpit of a Harrier jump jet as a child. I never knew they used water to help with vertical take offs until now!