Think of retargeting like cyberstalking. Someone performs an action (often a visit to your site) & has a cookie placed on her or his browser. Then as they go visiting other sites around the web, your ad appears in front of them, as a banner or other type of display ad, on whatever sites they visit – so long as that site accepts ads from the ad network you use for retargeting.
Retargeting can be done through various ad networks & platforms.