A 503 tells the browser the server is alive but temporarily can't handle the request, whether that's because it's overloaded with traffic, down for scheduled maintenance, or running short on bandwidth. It's meant as a signal that the situation is short-term and normal service will resume.
That temporary framing matters for SEO specifically. Search engines that see a 503 generally treat it as "come back later" rather than "this page is gone," and won't drop the page from the index the way a sustained run of 404s or 500s eventually would. That's exactly why 503 is the correct code to serve during planned maintenance, ideally paired with a Retry-After header telling crawlers roughly when to check again, rather than taking a site fully offline or, worse, letting maintenance pages return a 200 that gets indexed as real content.
