Site retargeting is a way for websites to show ads to people who have visited their site before. It works like this: when you visit a website, the website puts a tiny picture (called a cookie) on your computer. Then, when you visit other websites, the other websites can recognize the cookie, and they can show you ads related to the website you visited earlier. So if you visited a website about shoes, you might see a lot of ads for shoes on other websites.