In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties.
Chemical substances are a purified form of matter that is characterized by a constant chemical composition and set of properties (e.g., their boiling point, density, melting point, etc. at fixed atmospheric pressure). Molecules are a type of chemical substance.
An example of a molecule and hence a chemical substance too, is water which is composed of two hydrogen atoms bound, covalently, to a single oxygen atom, it fits the definition of a chemical substance because regardless of how many times you try to divide, say, a glass of water into ever smaller quantities the chemical composition (i.e., two hydrogens for every one oxygen) will stay the same, likewise its properties under the same conditions will remain identical.