The “Save-Data” header field is a boolean that, in requests, indicates client’s explicit opt-in for reduced data usage, due to high transfer costs, slow connection speeds, or other reasons. When communicated to origins, it allows them to deliver alternate content honoring such preference - e.g. smaller image and video resources, alternate "light mode" markup, etc.
Docs: https://developers.google.com/web/updates/2016/02/save-data?hl=en