In everyday life the word pure usually means natural, or with nothing added. A carton of pure orange juice is still a mixture of water, sugars, acids and flavour compounds. Mineral water is sold as pure, but it contains dissolved minerals.
In chemistry a pure substance is a single element or a single compound, with nothing else mixed in. Distilled water is pure because it contains only water. A mixture contains two or more substances that are not chemically joined, so a mixture is impure in the scientific sense, even if every part of it is natural and harmless.
A solution such as salt dissolved in water is still a mixture, because the salt and the water are both still there. Chemists check purity by measuring a property such as melting point and comparing it with the data book value. Purification of a compound removes the unwanted substances to leave a sample that is as close to pure as possible.