I'm not quite sure how to read this graph? What does TDH (Total Dynamic Head) mean?
Head is another word for pressure. The units are convenient: if you are pumping water uphill 20 ft, then the static pressure you have to pump against is 20 ft of head. 1 ft of head = 0.433 psi.
The static part of that statement is for the case water isn't flowing or is flowing so slowly you can ignore any dynamic effect. In practice when pumping significant gpm through a fairly small pipe, the water flow sees drag from the pipe and that creates extra pressure you have to pump against. So dynamic head, for a given flow rate through a given piping configuration, would be how much extra pressure that drag creates, which your pump has to overcome. Dynamic head strictly increases with increasing flow rate.
Total dynamic head is the sum of dynamic head and static head. If the graph you posted is from some online app that required you to put in some info about the static head and about the piping configuration, then the blue line on that graph shows what the total dynamic head is for a given flow rate through that system (if your input are accurate). It is an increasing function, as again dynamic head increases with flow rate.
Now a pump is going to be able to push water/sewage with a certain amount of oomph. Meaning it can push a lot of water against a small head, or less water against a bigger head. So if for the pump you graph "pressure the pump can push against" vs "the flow rate the pump can move at that pressure," you get a graph where head decreases with flow rate. That's the pump curve for the pump.
If you graph the pump curve and the system curve on the same graph, the point where they cross tells you where they will jointly operate if you connect that pump to that system. It's the point of equilibrium, where the pump can handle creating that much pressure at that flow rate, while the system itself requires that much pressure to allow that much flow to move through it.
Your graph has multiple pump curves, and the intersection between any one of them and the system curve (the one marked TDH) tells you what that pump will do. So for example the orange line and the blue line cross at about 15 ft of head and about 55 gpm (estimating from the graph). If you're happy with a pump that can move 55 gpm, you can go for that orange pump. At the other end of the options shown, the green line and the blue line cross at about 52 ft of head and 115 gpm. So if you want 115 gpm of flow, you'd use the green pump. And then you have various choices in between.
Note that with the larger pump and the higher flow rate comes a higher head of 52 ft. That means if you choose that, the pump will pressurize the pipe connected to it at 52 ft of head. So that might require a sturdier pipe able to resist 52 ft of head than if you chose the smallest pump that only would be making 15 ft of head connected to your system.
Cheers, Wayne