constituent quark
Here is a puzzle. The up and down quarks are extremely light — a few thousandths of a proton's mass each. Yet if you treat a proton as three quarks and ask how heavy each one effectively is, you get about a third of a proton's mass, which is hundreds of times bigger. How can the same quark be both featherlight and heavy? The answer is the idea of the constituent quark.
A constituent quark is a quark together with the cloud of gluons and sea quarks that surrounds it inside a hadron, treated as one effective object. The tiny number physicists call the quark's true (or current) mass is what the quark would weigh stripped bare. The constituent mass is much larger because it bundles in the energy of the strong-force cloud the quark drags along. Picture a light marble rolling through thick syrup: it behaves as if much heavier because it carries the syrup with it.
The constituent-quark picture is an approximation, not a fundamental truth, but it is a remarkably useful one. By treating a baryon as three heavy constituent quarks and a meson as two, physicists can predict the masses, charges, and magnetic moments of many hadrons with surprising accuracy, using little more than simple addition. It is the practical engine behind the simple quark model, even though the deeper reality is the messy, swirling interior described by quantum chromodynamics.
A current up quark weighs only about 2 MeV, but its constituent mass inside a proton is roughly 300 MeV — about a third of the proton, made up mostly of strong-force energy.
The same quark has a tiny "bare" mass but a much larger effective mass once it drags its strong-force cloud along.
Constituent and current mass are two ways of describing the same quark, not two different particles. The constituent number is a convenient effective figure that quietly absorbs most of the proton's strong-force binding energy.