Answer:
A hydraulic accumulator contains a bladder filled with a compressible gas, usually nitrogen. The pressure of the gas in the bladder is known as the pre-charge, and will vary based on the ambient temperature. Hydraulic oil is pumped into the accumulator but outside of the bladder. As the oil is pumped in, the bladder compresses, which exerts a force on the oil. There is usually an pressure transducer in the system which will signal the hydraulic pump to turn off when a certain oil pressure is reached in the accumulator.
A hydraulic accumulator can have several uses. It can be used to store hydraulic pressure for later use. It can be also used as a type of "shock absorber" for hydraulic systems.