The "Last-Modified" header is an HTTP response header used by web servers to indicate the last time a resource, such as a webpage or image, was changed. This information helps browsers and other clients determine whether they need to fetch a new version of the resource or if they can use a cached version, improving efficiency and reducing unnecessary data transfer. When a client requests a resource, it can include an "If-Modified-Since" header with a date. If the resource has not been modified since that date, the server can respond with a 304 Not Modified status, signaling that the cached version is still valid. This process enhances web performance and user experience.