What does pure substance mean

Pure substances are defined as substances that are made of only one type of atom or only one type of molecule (a group of atoms bonded together).

An element is a pure substance that cannot be separated into simpler substances by chemical or physical means ... Pure substances are often called pure to set them apart from mixtures. A substance can be anything. It doesn't have to consist of a single element or type of molecule. Pure hydrogen is a pure substance.