Complex sentence: In thermodynamic equilibrium, a cavity in a body contains Planckian radiation, provided that its walls are opaque and do not move; beyond that proviso, there is no restriction on the material composition of the walls; in this sense, the Planck distribution is universal.
Split into simpler sentences: In thermodynamic equilibrium, a cavity in a body contains Planckian radiation, provided that its walls are opaque and do not move. There is otherwise no restriction on the material composition of the walls; in this sense, the Planck distribution is universal.