Without pressure, the fluid doesn't move. Every fluid system has what is called a "system head curve" ..... It starts at 0,0 as at no flow, there's no pressure.....as flow increases, the fluid moving past restrictions causes back pressure. This resistance increase at a rate at a geometric rate with the pressure increase being approximately proportional to the increase in flow to the 1.85 power.....(google Hazel Williams equation if the urge strikes ya for more info)..... so double the flow and ya increase backpressure by 3.6
So this curve starts out prtty flat but quickly rises in slope based upon the ratio above. The pump curve starts at what is called the "shut off head' ....this is at the 0 flow point and is the point at which the pressure is high enough that the pump can not move any water against it. And unlike the system curve which goes up and it moves to the right, the pump curve drops as it moves to the right. At some point the curves intersect and that is your operating point. Martin gives a good example here:
Each restriction, each fitting, each change in direction, even each air bubble, adds backpressure or "head loss".....the pump will pump as much as it can until the pressure it can provide is an exact match to the head loss in the system.