Retargeting is one of the best online advertising tools available on the market today.
Retargeting makes it possible to reach Internet users again after they have visited certain pages of your website or have seen an online advertisement banner  on third party websites. A good strategy and monitoring helps you to define who should be targeted again, how many times and with which message(s).

This video from our U.S. counterparts at Audience Archer explains the principle in a very clear way.