If you browse an online webshop and then, later on, see an ad for that webshop on an unrelated site, it’s because you have been retargeted. A retargeting pixel had been placed on that store’s website, and when you left, the pixel sent signals to a third-party platform to serve the store’s ads on other websites you visited afterwards.
Retargeting allows advertisers to target people whose behaviour has indicated an interest in their product/service – essentially, the visitor is identified as a ‘sales lead’, and the retargeting ads are sent out to close that sale.
Retargeting raises brand awareness and potentially encourages visitors to return and make a purchase. See Pixels.