what is hydraulic pressure ?

Pressure control is achieved in hydraulic systems by metering the flow of a fluid into or out of a constrained volume. Relief valves and pressure-reducing valves are not pressure controllers. They limit or reduce pressure, but they do not really control pressure to a desired value. Pressure-reducing valves can only reduce pressure, and only by a set ratio. The output pressure is limited by the input pressure. Relief valves only limit pressure to a set value. Another limitation of these types of devices is that they use springs and are only proportional-control devices. They have no rate control or ability to change different pressures on the fly.
It’s All About Energy Adding fluid to a fixed volume increases the pressure, whereas letting fluid out decreases pressure. There is a misconception that pressure is controlled by the pressure gain curve of the valve. This is true only in a testing application, where pressure sensors are connected directly to the A and B ports of the valve. There is no volume of oil under compression.
Another misconception is that “pressure is resistance to flow”. It would be better to say that a resistance to flow will causes a drop in the pressure. Yet another problem is that pressure is related to the internal energy of the fluid. A resistance to flow does not add energy, but it will dissipate energy in the form of heat. Still another widely held misconception is that pumps generate flow, not pressure. Pumps convert electrical energy to mechanical energy, then to hydraulic energy. For a fluid to have energy it must be under pressure, with potential energy due either to elevation or velocity. A pump adds energy to oil in any of the three ways, as described by Bernoulli’s equation:
where P is the pressure,
ρ is the density,
v is the velocity,
g is the acceleration due to gravity, and
h is the elevation.
All three terms involve energy. The velocity and gravity term have a density element that makes them an energy density term directly. The pressure still has units of psi, but this can be converted to energy by multiplying by a volume:
Now multiply by a cubic inch:
The unit of pounds force-inch (lbf-in.) are units of energy. These units can also be converted to BTU, which is less inconvenient for us.