Above is a chart I borrowed from Aquabee pump. The 3 colored lines shows 3 of their pump models.
Firstly, the pump sucks water in from one hole(i.e. inlet) & pushes the water out the outlet.
The horizontal line from the above chart shows that pump UP6000 can push water at a rate of 6000 liters per hour(l/h). But if you were attach a pipe to the pump outlet & point it upwards, the water flow is going to start slowing down because it has to 'fight' gravity. Hence, the longer the vertical pipe, the slower will be the water, until it totally stops flowing. For UP6000, the chart shows that its flow rate stops (i.e. 0 l/h) at a height of 5 meters.
In a typical display tank & sump setup, you sump usually sits below the main display tank. Thus requiring the return pump to push water upwards back into the display tank. The height is typically about 1.5 meter. So if you're using UP6000 as your return pump, at 1.5 meter, you should be getting a flow rate of about 5000 l/h. This 1000 l/h of water flow missing is the head loss.
In addition, if you have a few piping elbows along the way to reach the main display tank, the flow would reduce a little more.
Hope the explanation is clear enough.