Mould is the British spelling for "mold" — the stuff you find growing on old bread or the container you use to create your jello masterpieces.
Living mould is like “mildew,” it’s a type of fungus that grows on damp or decaying matter. Mould can also mean a “caste” used to shape a hardening liquid. In this sense mould can refer to both the container and the end product that comes out of the mould. In addition, mould is used as a verb to describe the process of making a mould. When you pour plaster into a mould to make a shape, you mould that shape.