The "mass gap" refers to the difference in mass between the lightest particle in a quantum field theory and the next heavier particle. In many theories, there is a minimum mass that particles must have, creating a gap where no particles exist. This concept is important in understanding the stability and behavior of particles in physics. In the context of quantum field theory, the mass gap helps explain why certain particles, like gluons and photons, do not have mass, while others, such as higgs bosons, do. The existence of a mass gap can influence the dynamics of particle interactions and the formation of hadrons in quantum chromodynamics.