The customers who almost bought
Here's an uncomfortable reality of every website: the large majority of people who visit leave without doing anything. They read a bit, maybe liked what they saw, then got distracted, weren't quite ready, or wanted to think it over — and left. In cold advertising, those visitors are gone, and you paid to acquire every one of them.
Retargeting (also called remarketing) refuses to let them vanish. It shows ads specifically to people who already visited your website, following up with a reminder as they browse other sites and scroll social media. Instead of starting over with strangers, you're re-engaging people who already raised their hand by showing up once.
This is why retargeting is some of the most cost-effective advertising there is. You're not paying to build awareness from scratch — that already happened. You're paying to close the gap between "interested" and "ready," which is a much shorter, cheaper distance to travel.