The Delta P is the pressure drop in a heating system which can be defined as the difference in total pressure between two points (e.g A and B) of a fluid carried through the pipework or heating equipment such as a radiator as shown below.

Pressure drop occurs when frictional forces, caused by the resistance to the water flows through the pipe or radiator etc.

The main causes of resistance to the flow of water is the fluid velocity through the pipe or vessel and the fluid viscosity.

Pressure drop increases proportional to the frictional resistance within the heating pipework and heating equipment installed.
A heating system with a large amount of pipe fittings, tube convergence, divergence, turns, surface roughness and other physical properties will affect the overall system pressure drop.

High flow velocities and / or high fluid viscosities result in a larger pressure drop across a section of pipe, pipe fittings or heating equipment.

Low velocity will result in lower or even zero pressure drop.

OVERCOMING THE PRESSURE LOSS BY USING A PUMP
The head or pressure can be overcome by a pump but it must be sized to be at least equal to the total resistance to the flow of water in the whole of the heating circuit.