StellarMixtures are combinations of two or more substances that are not chemically bonded to one another. Because they are not bonded, they can be separated again by physical means. For example, salt dissolved in water is a mixture because the salt can be recovered by removing the water. Mixtures can be either heterogeneous or homogeneous. A heterogeneous mixture is not uniform throughout, while a homogeneous mixture has a uniform composition.
The main idea is that a mixture keeps the identities of the substances that make it up. Since the substances are physically combined rather than chemically changed, their properties are still present and can often be used to separate them later. This is why classifying a mixture as homogeneous or heterogeneous matters: it tells you how uniformly the substances are distributed and what the mixture looks like at the macroscopic level.