Note the following definitions:

M' = Geostophic Mass Transport = V_g delta n (3.5)

delta z = - (rho) delta h (3.6)

where rho is seawater density (greek letter rho) and delta h is steric height

Put (3.5) and (3.6) into (3.4) and you get

V_g = - g/f (delta z/delta n)

Look familiar? In atmospheric geostrophic flow, the usual explanation is that air parcels begin "rolling" down the height gradient towards lower values (due to gravity) and then get deflected to the right by Coriolis acceleration. In the diagram below, you can see the same balance (except the author called the pressure gradient acceleration shown in red "gravity" but it is really a pressure gradient accleration since gravity acts at right angles to the surface.)