This is the order of reactivity of (some) metal elements. The metals at the top are more reactive, and the bottom are less reactive.

The series, along with a handy mnemonic, goes as follows:

potassium         Please
sodium            Send
lithium           Little
calcium           Charlie
magnesium         McLein
aluminium         A
zinc              Zebra
iron              If
tin               The
lead              Lean
hydrogen          Horse
copper            Can't
mercury           Munch
silver            Sweet
gold              Grass
platinum          Peacefully