Did you ever wonder how ads showing the same product that you have browsed few hours or days back?. This is because of remarketing. Remarketing is a way of advertising in which publishers can show their ads to users who have previously visited their website or product. As per studies, remarketing will give you much more ROI (Return Of Investment) than any other ad targeting.
Suppose you are looking for an iPhone in amazon store, and unfortunately before purchasing it you abort the plan. Guess if next day amazon shows you ad of the same phone on pages you browsing, definitely it will makes you rethink about the product and most probably you will buy it. This is how remarketing perform for online businesses.