A pure substance has the same chemical composition throughout samples of that substance. Pure water consists of H₂O molecules. Saltwater contains water and dissolved salts, and its salt concentration can change from one sample to another. That makes saltwater a mixture even when it looks uniform.
“Pure” is a classification of composition here, not a claim that something is safe or healthy. A pure substance may be an element or a compound. The distinction helps you ask whether a sample is one chemical substance or a combination whose proportions can change.