Retargeting is a way for large ad networks to serve the same ad to the same person multiple times and in different locations. Facebook and Google are the two biggest ad networks that use retargeting and you've probably noticed it in action when you visit a website and then see an advertisement for the same thing after you leave and go to another website. It is a way to track your visitors and show them ads that they are more likely to be interested in, which makes them more likely to buy later on.
You go to HomeDepot.com and look at a snowblower but then decide that it is too expensive. You leave and go over to Facebook to check in on your friends. As you're talking with your friends, an ad for that same snowblower pops up on Facebook. As you continue to surf the internet, the same ad keeps popping up, reminding you to check it out.