Complex sentence: The down quark is very light compared to nucleons (~ 940 MeV/c²); most of the mass of nucleons comes from the energy in the gluon field holding the quarks together, rather than from the quark masses themselves.
Split into simpler sentences: The down quark is very light compared to nucleons (~ 940 MeV/c 2). The reason is that the majority of the mass of nucleons comes from the energy in the gluon field holding the quarks together, and not the quark masses themselves.