The central product of every hydraulic system is the hydraulic pump.
The main function of the pump is to convert mechanical energy (rotational force, torque) into hydraulic energy (fluid pressure, fluid flow).
Hydraulic pumps are Positive displacement pumps. These work with sealed chambers through which the hydraulic fluid flows from the inlet side (suction connection) to the outlet side (pressure connection).
There is no direct connection between the suction and pressure connections, which is why hydraulic pumps work well even at high pressure.