Retargeting (also called remarketing) shows ads to people who already interacted with you, most often visitors who came to your site but didn't convert. Because the audience is already warm, it is usually far cheaper per result than prospecting, and on Google you can keep retargeting a visitor for up to 540 days.
Why it is the cheapest pipeline
You already paid to bring a visitor in through Search or another channel. If they leave without converting, that spend is wasted unless you re-engage them. A retargeting audience is warm, so click and conversion costs are typically a fraction of cold prospecting, often cents on the dollar versus a Search click.
How to run it
Build an audience of site visitors (or specific page viewers, cart abandoners, past leads) and serve them a dedicated Display or video campaign. On Google, remarketing list membership lasts up to 540 days. It is the third leg of the three-campaign B2B setup.