There is a lot of information on flow rate and pressure. Flow rate is simply how much fluid passes a point in a given amount of time. This is usually litres per second or L.P.S.. The flow rate dictates how much pressure there is inside the system. When valves open the fluid begins to flow, just like you turning on the water at home. As the fluid flows the pressure decreases. This occurs when you are taking a shower and someone else turns on the water. The flow rate, the amount of water moving, increases and you experience a drop in pressure at your end.
Valves are merely doors that open and close to let fluid flow. The more the valve is open, the greater the flow rate, thus less pressure. The more a valve is closed, the flow rate decreases, but the pressure increases. You can try this with your garden hose at home without a nozzle on the end. With the water on, put your thumb over the end. Less water will come out but you will feel the hose firm up. The pressure increased as you restricted the flow.
Valves are merely doors that open and close to let fluid flow. The more the valve is open, the greater the flow rate, thus less pressure. The more a valve is closed, the flow rate decreases, but the pressure increases. You can try this with your garden hose at home without a nozzle on the end. With the water on, put your thumb over the end. Less water will come out but you will feel the hose firm up. The pressure increased as you restricted the flow.