Compounds are substances that are made of two or more atoms via a chemical reaction. The atoms within a compound are attached via a chemical bond. The ratio of atoms within a compound can never change. For example, H2O (water) will always contain two hydrogen atoms for every one oxygen atom.
On the other hand, mixtures are not chemically bound. They are physical combinations of two or more substances. The substances within the mixture maintain their physical and chemical properties. No new substances are made via a chemical reaction. The proportion (ratio) of the ingredients within a mixture can change. For example, salads are mixtures. My salad may look very different than your salad in terms of what you and I use as ingredients within our separate salads.