Remarketing allows advertisers to target ads to visitors who previously visited their website but did not make a purchase. It works by placing cookies on visitors' devices to track them as they browse other sites, then showing them targeted ads. Remarketing ads can be images, text, or animated videos in various sizes. It provides flexibility to target specific audiences across different sites in a cost-effective way to remind visitors and increase brand awareness. Certain thresholds and criteria must be met to create effective remarketing campaigns.