What is reactivity series? How does the reactivity series of metals help in predicting the relative activities of various metals?

The arrangement of metals in the vertical column in the order of decreasing reactivity is called reactivity series or activity series. When a metal placed above hydrogen in the activity series, it means it will displace hydrogen from acids or water. Secondly, when a metal placed at the top (of the activity series), it would displace metal below it. In brief, a more reactive metal displaces a less reactive metal from its salt solution.