A mixture is made of two or more intermingled substances that do not combine. So you might have a mixture of sugar and salt. A mixture can also happen between a liquid and a solid or two liquids. That is called a solution. Salt dissolves (becomes dissolved in) water. But it never changes and can be recovered by evaporating the water. A mixture can also be between gases. Air is a mixture of gases, one of which is the oxygen that we breath in. Our lungs separate it from the other gases.

If a mixture is made of one material that is small and the other which is large, you can separate them out using a sieve - a filter. But most mixtures are very difficult to separate. Think, for example, of how difficult it would be to separate out sugar and flour.