Nambu-Goldstone bosons are a type of particle that arise in quantum field theory when a continuous symmetry is spontaneously broken. When a system has a symmetry that is not reflected in its ground state, these bosons emerge as massless excitations. They are named after Yoichiro Nambu and Jeffrey Goldstone, who contributed to the theoretical understanding of these particles. These bosons play a crucial role in various physical phenomena, including superfluidity and superconductivity. In these cases, the breaking of symmetry leads to the emergence of collective excitations, which can be described by Nambu-Goldstone bosons, helping to explain the behavior of the system at low energies.