The reactivity series lists metals from most reactive to least reactive: potassium, sodium, calcium, magnesium, aluminium, (carbon), zinc, iron, (hydrogen), copper, silver, gold. Carbon and hydrogen are not metals, but they are put in brackets to give a useful comparison.
The order comes from how vigorously the metals react with water and with dilute acids. Potassium is at the top, so it reacts violently with water. Gold is at the bottom and does not react with either.
These reactions show the relative tendency of metal atoms to form positive ions, called cations. When a metal reacts, its atoms lose electrons. The more reactive the metal, the more easily its atoms lose electrons, so the greater its tendency to form cations. Potassium atoms lose an electron very easily. Gold atoms hardly do at all.