In chemistry, what is the difference between a homogeneous mixture and a substance

It's pretty basic, but I don't understand yet.

Well, a (pure) substance consists of an entity/matter in which only the particular type of matter you are talking about exists, and does not vary from sample to sample. For example, suppose you had a PURE bar of gold. At the microscopic scale, you would find ONLY gold atoms, and no other kind. Or if you had a flask stoppered with nothing but CO2, then you would find ONLY CO2 molecules, NOTHING ELSE.

On the other hand, samples of matter from mixtures contain OTHER kinds of atoms/molecules/ions. For example, air is a mixture of OTHER kinds of atoms/molecules, such as nitrogen, oxygen, argon, helium, neon, carbon dioxde, water vapor, etc.

Thus, a mixture contains DIFFERENT kinds and number of atoms/molecules/ions from sample to sample, while in a pure substance the kinds and numbers of atoms do not differ from sample to sample.

homo mixture means that they have blended together completely and you can't visually differentiate between them. according to wikipedia a substance is any material with a "definite chemical composition"

A mixture is made up of substances

Homogenous mixture is like sugar, blood, kool aid etc. mixtures consists of two substances.

A substance is an element or a compound.

So if you combine the substances Salt + Water you get a homogenous mixture called Salt water.