Take a boy chatting up the new girl in school(boy = metal, girl = nonmetal), then the bully of the school comes over and pushes the boy out of the way and starts chatting up the girl. Its not fair, but its the truth.
Displacement reactions are exactly the same! Take a compound like copper oxide the oxide is quite happy with the copper until a bully, Zinc (for aguments sake but it could be any metal higher up the reactivity series) comes and pushes "displaces" the less reactive copper out.
zinc + copper oxide → zinc oxide + copper
or in symbols
Zn + CuO → ZnO + Cu
Question: Which other metals could displace Copper (Cu)?
Answer: Lead, Tin, Iron, Aluminium, Magnesium, Calcium, Sodium or Potassium.
Note: Any metal above the metal in the reactivity series can displace (push) the metal in the compound.