Ans: QCD is a type of quantum field theory that helps describe the behaviour of sub-atomic particles. This theory describes the action of the strong force.Talking about the role of QCD in theoretical physics, it is the theory of the strong interaction between the hypothetical massive sub-atomic particles like quarks and gluons, and the fundamental particles that form the composite hadrons, like the proton, neutron, and pion.The QCD analog of electric charge is a property called colour, where it bears two properties, viz: Colour confinement, and Asymptotic freedom.