Google Remarketing can help you reach people who have previously visited your website. You can show them some specific ads that are tailored to them based on which sections of your site they visited. Your ads could appear to them as they browse other sites that are part of the Google Display Network (that includes more than 2 Million websites including Gmail, YouTube, Gulf News, Khaleej Times, Dubizzle and more) or as they search for terms related to your products on Google.
These two videos can help you understand remarketing in a better way:
The infographic by Google also helpful to understand how remarketing works:
Find more details here: https://support.google.com/adwords/answer/2454000?hl=en