Can you describe how mixtures relate to substances?

A substance in chemistry is a form of matter consisting having fixed chemical composition (like water). A mixture is a material made up of two or more substances (like water with sugar) - mixed, but not combined with chemical bonding.

A substance is a pure form of matter in which all the constituent particles are the same in their chemical nature, while a mixture is a combination of two or more substances. A substance cannot be separated into other kinds of matter by any physical process. But a mixture can be separated into pure substances  by appropriate separation techniques such as filteration, sedimentation, evaporation, crystallisation, etc..