Centrifugal pump start

A centrifugal pump will deliver a certain head against system back pressure, say 100m.

If full of air, the pressure developed will be, say, 100m of air - not very much pressure and insufficient to push it's way out of the pump and so draw liquid in. We say the pump is not self priming for that reason.

A self priming pump has a space where liquid partly covers the impeller so it can be thrown out and entrain the air in the pump at the same time. If it doesn't pull out enough air to prime the pump, the liquid falls back into the impeller, entrains more air, etc.

The above is a very much simplified version of what happens but I hope it will provide some clarification.