This photo shows hydraulic booster used in HPHT Hydraulic Press. I could not understand its function. How does this pump boost the hydraulic pressure ? Thanks.
A Hydraulic intensifier, also called booster, works by converting hydraulic power from a lower pressure input to a higher pressure output with lower volume (flow). Wikipedia
A simple design is two connected pistons of different diameter. The lower pressure fluid is applied to the larger piston, the resulting force is transferred to the smaller piston which applies it to it's fluid. Since the second piston is smaller the force will result in a higher pressure. Since the pistons are connected, they have the same stroke meaning that the smaller piston will move a smaller volume of fluid resulting in a lower flow. This is the working principle, you will also need some valves to control the input and collect the output.
In your picture the electric motor will drive a pump that produces the lower pressure, this lower pressure is feed into the intensifier which in turn produces the higher pressure.