Cookie sync or matching is a process that is used to link data from different sources that is associated with the same user. This is typically done by using cookies, which are small pieces of data that are stored on a user's computer or mobile device by a website. Cookies can be used to track a user's browsing habits and preferences, and can be accessed by the website that set the cookie, as well as by other websites that have been granted access to the cookie.

In the context of advertising, cookie sync or matching is often used to link data from different ad platforms or networks that are associated with the same user. This can help advertisers to get a more complete picture of a user's behavior and interests, which can be used to serve more relevant and effective ads. Cookie sync or matching can also help to prevent the same ad from being shown to the same user multiple times, which can improve the user experience and the performance of the ads.