One of the key targeting options in Google Display Network is called Remarketing. This allows you to show a customized message to users who have visited your site before but chose not to purchase.
How is this useful? Users rarely purchase anything the first time they visit your site. Instead, they think about the services or products you offer and consider whether they want to take action. Remarketing reminds these users about you and builds up their trust in your brand over time. This can yield the highest conversion rates when compared to all the other targeting options.