The radiation zone of a star (e.g., the Sun) is that interior region where energy is transferred outward through radiative diffusion, energy being passed through material by radiative transfer, via electromagnetic radiation even though the material is opaque: photons in the radiation zone are absorbed by the material's atoms, exciting them so photons are subsequently emitted in a random direction, such a resulting random path of absorbed/emitted photons termed a random walk. This results in a slow outward transfer of energy, as some photons randomly reach the outside of the star and escape with their energy, and others randomly replace them, and so forth. The number of steps in such a trail of absorption/emission is incredibly high and calculated as requiring thousands of years for a photon entering the Sun's radiation zone to escape.
The term radiation zone (or variants) is also sometimes used for such a portion of planets as well, such as gas giants (i.e., planets like Jupiter or Saturn). Its internal energy may be from radioactive heating or tidal heating and/or may be left over from planet formation.