Yes, a noob Q, but that's ok.
It's confusing because many different terms are used by various manufacturers. The short answer is that it's the maximum pressure that the pump can deliver.
The long answer is...
It's sometimes called "dead head", because it's the pressure level at which the flow rate becomes zero.
It's also called "head" or "max head", as this max pressure is often expressed as the maximum height of a water column that a pump can sustain.
If you look at any of the PQ graphs above, you'll see a maximum flow rate, and a maximum pressure. Obviously, once you hit the max flow, there's no pressure, and when you hit max pressure, there's no flow...