Radial symmetry means (biologically) that the animal structures are situated in a radial or circular pattern around a center point with nonexistence of sides, like right or left. An alternative type of symmetry in which structures are placed equally in the sides of a longitudinal axis is the lateral symmetry (the symmetry present in human beings, for example).
Chordates present lateral symmetry.
Besides cnidarians another animal phylum with species presenting secondary radial symmetry is the phylum Echinodermata. (It is considered that the simplicity of poriferans does not characterize any symmetry.)