You have two methods of putting hydraulic fluid into a system: closed and open. Closed systems require a pressure system to keep the pressure up, open ones use a pressurized hydraulic that is expended. The open system was used in this situation to reduce the weight necessary to run a closed system.

The hydraulic liquids were used to adjust the fins seen in the video on this page: http://www.businessinsider.com/spacex-problem-falcon9-crash-...

Elon Musk replied that the team had underestimated the amount necessary by 10%.