Ads shown to people who've already interacted with your brand. Following them around the internet.
Retargeting shows ads specifically to people who've visited your site, watched your video, or otherwise engaged. They're already somewhat aware, so they convert much higher than cold audiences. The cost: they can feel 'stalker-y' if overdone. Smart retargeting is frequency-capped and segmented (different messages for different funnel stages).
Retargeting typically has the best ROAS of any paid channel, because you're spending on people who are already warm. But it's a closing tool, not an acquisition tool; it can't grow the funnel, only convert what's already there. Balance with acquisition channels.