Vary headers are HTTP response headers used by web servers to indicate which request headers were considered when selecting a response. By specifying headers such as Accept-Encoding, User-Agent, or Cookie in the Vary header, servers inform caches (like browsers or CDNs) that different versions of the response may exist based on those request headers. This helps ensure that users receive the correct content variant, such as compressed or language-specific pages, and prevents caching issues where the wrong version might be served to other users. Proper use of Vary headers is essential for effective content negotiation and reliable web performance.