What does ‘Serve static assets with an efficient cache policy’ mean?

This warning means your site is asking returning visitors to re-download files - logos, stylesheets, scripts - that have not changed since their last visit. Browser caching tells them to keep a local copy instead:

• How caching works - your server sends a header telling the browser how long it can reuse a stored file before checking again.
• Why it matters - without it, every page load re-fetches everything, which is slow and wasteful, especially on repeat visits.
• The fix is a server setting - set a long cache lifetime on files that rarely change, like images, fonts and compiled CSS or JavaScript.

This is a one-time configuration on your host or CDN, not an ongoing chore, and it makes repeat visits noticeably faster. Ask your host or developer to set a long cache policy on static assets - it is set-and-forget.