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, especially in quantum field theory, this gap indicates that there are no particles with masses in between, leading to a clear distinction in the spectrum of particle masses. This concept is significant in understanding the behavior of elementary particles and the stability of vacuum states. The mass gap helps explain why certain particles, like hadrons, can exist without lighter counterparts, influencing the dynamics of particle interactions and the formation of matter.