The chart represents the loss of flow that the pump will lose as it pumps water against a vertical run of pipe, hose or tubing.
Sort of like pumping water uphill.
At zero feet of head - such as if you were take the pump out of the box, place it a tub of water and plug it in - about 1200 gallons per hours will shoot out of the pump's outlet.
If you hook a pipe or hose up to this same pump and the water must be pumped up 2 feet vertical - the pumps flowrate will be decreased to around 1150 gallons per hour.
Same for 4 feet = flow drops to around 1100 gph, and so on.
This drop in flow is natural when using a centrifugal pump.
The loss of flow is due to the inefficiency of the pump being able to pump the same rate when there is vertical head pressure above the pump.
Some will call this backpressure, although most call it head pressure since it's truly a vertical head were talking about.
Putting a restriction in the hose, or say dropping from 1" piping down to 3/4" piping will induce a pressure drop into the equation.
This could be described as inducing some backpressure on the pump.
Most pumps come with a chart that shows how it performs ( or should perform ) under various "head" pressure installations.
All you must do is grab a tape measure, and measuer the distance from the pump's outlet to the return hose outlet in your tank ( to the highest point in the return line piping ) and value willl be your head.
Take this distance, say it's 36 inches ( 3 feet )
You'd have 36 inches or 3 feet of head pressure ( freshwater )
Take this 36 inches times your specific gravity say 1.024 and you're actual head is 36.86 inches of head pressure.
Not really worth including the saltwater's density when determining the head, but with other heavier or lighter liquids, it along with the temperature of the liquid can make a huge difference when selecting pumps.
The Mag12 is listed as having a 15 feet maximum head. In other words, if you try to pump water up higher than 15 feet - it will not work so well. The vertical distance or head pressure becomes too much for the pump to overcome, and you will get little if any water out the top of this 15 foot vertical length of pipe.