A substance is matter which has a specific composition and specific properties.
Every pure element is a substance. Every pure compound is a substance.
Examples of substances: Iron is an element and hence is also a substance. Methane is a compound and hence is also a substance.
Examples of non-substances: Salt water is not a substance. It is a mixture of two substances - sodium chloride and water. Its composition and therefore its properties are not fixed. Gasoline is not a substance. It is a mixture of hydrocarbons and, depending on the composition of the gasoline mixture, gasoline's properties can vary.