Remarketing is targeting ads to people who’ve already visited:
- multiple websites on the Display Network
- your website after they’ve searched on Google
- competitors’ websites multiple times
- your website as they browse websites and use apps on the Display Network
Explanation: Remarketing lets you show ads to people who have visited your website or used your mobile app before. When people leave your website without buying anything, for example, remarketing helps you reconnect with them by showing relevant ads as they browse the web, as they use mobile apps, or as they search on Google.
Read more here: https://support.google.com/adwords/answer/2453998?hl=en