Yang–Mills existence and mass gap

Yang-Mills theory is a type of physics that describes the interactions between different kinds of particles. It is important because it helps us explain things like why some particles have mass (given off by energy) and others do not. The Yang-Mills existence and mass gap means that there is a certain amount of energy that is necessary for a particle to have mass. If the particle does not have at least this amount of energy, then it won't have mass. This is because the Yang-Mills theory describes the particles' interactions and the energy needed for them to have mass.