Have you visited an online store, looked at a product, and then found that when you go, for example onto Facebook, you get ads for the same product? How does that happen?
If you use remarketing you will more easily be able to reach those who have already seen your products through targeted advertising.
Remarkerting is also called "behavioral advertising". It is based on customers' past actions. If a customer has visited your site ad looked at a product, you can buy an ad to show this in other media outlets. In this way you reinforce interest in the product.