Nucleophilic substitution reactions are reactions that is studied mostly in organic chemistry, where organic compounds will undergo a reaction very similar to the double displacement reaction, where a nucleophile (for example: halogens, hydroxide ion, cyanide ion, ammonia), will "attack" the organic molecule and switch places with the nucleophile from the organic molecule.
One example for this reaction is:
Propyl bromide (C3H7Br) + Potassium azide (KN3) = Propyl azide (C3H7N3) + Potassium bromide (KBr)