I think plateau here means a condition in which a (pure) substance reaches stability.
For example, if you have table salt (NaCl), it'll stay being table salt if you don't alter the condition with chemical reactions.
If I recall correctly (and correct me if I'm wrong
), it's because of the bonding between elements.
All known elements except noble gases tend to reach stability (or have a noble gas structure) by either gaining or losing electron(s), which is often called ionic bond or even sharing electrons (called covalent bond).