A "walled garden" refers to a closed ecosystem where the platform owner controls all aspects of the environment, including the data and the technology used. Companies like Google, Meta , and Amazon operate walled gardens because they control and restrict the flow of data in and out of their platforms. These environments offer advertisers access to a large and engaged audience but limit the transparency and flexibility of data usage and analytics. The benefits of walled gardens include high-quality data, sophisticated targeting options, and brand safety, while the drawbacks include limited interoperability and challenges in measuring cross-platform performance.