Have you ever used a syringe for injection? You push the plunger with your thumb, and the liquid in the syringe shoots out from the needle. The greater the pressure your thumb exerts, the farther the liquid will be ejected and the lower the pressure. Shoot closer.
Then the injection pressure is equivalent to the pressure of your thumb on the piston. On the injection molding machine, the motor drives the oil pump to exert pressure on the screw.