In everyday life, pure often means that nothing has been added to something. Pure orange juice, for example, has no added sugar or water, but it still contains water, sugar, acids and many other substances.
In chemistry, pure has a stricter meaning. A pure substance is a single element or a single compound, with only one type of substance present and nothing else mixed in. Pure water contains only H2O and nothing else.
A mixture contains two or more substances that are not chemically combined. Air is a mixture of nitrogen, oxygen and other gases, and sea water is a mixture of water and dissolved salts. Because the substances are not chemically combined, each one keeps its own properties, and a mixture can be separated by physical methods such as filtration or distillation. A compound is different, because its elements are chemically joined and cannot be separated by physical methods.