Hydraulics systems are all about transmitting power.
They do this by applying force at one point in the system. It’s then transmitted to another point in the system using a fluid that cannot be compressed – typically an oil of some type.
For a basic hydraulic system, a pipe filled with oil has a rod with a piston connected at the end. Pushing oil into one end of the pipe pushes the rod down. Pushing oil into the other end pushes the rod up.