The cladding is the region of the fiber surrounding the core. In order to create guiding conditions for the light, the refractive index of the cladding must be lower than the core's refractive index.
Fused silica is the typical material of which the cladding is made in optical fibers but low-index polymers and fluorine-silica doped glass are also relatively common materials in double-cladding fibers and pure silica core fibers. In PMMA fibers both cladding and core would be made out of plastic materials.