Retargeting, also known as remarketing, displays personalized ads in online advertising campaigns. It engages users after they’ve bounced from your website and is based on their behavior, displayed during their visit.
By collecting data about the sites they’ve visited, actions they’ve performed and events they’ve triggered, you are able to create audiences/segments based on these and personalize the content.
The goal is to reach users, who haven’t performed desired actions and generate more Conversions, thus improve the Conversion rate.
More about retargeting on the Piwik PRO blog: