Poor resumes don't list any tangible accomplishments or lack the proper context to let the reader even figure out if the accomplishment was useful. If you read a job advertisement, that's a list of responsibilities - a resume should outline responsibilities (especially if the responsibility is critical), but also describe unique accomplishments.

That's the main thing, but there are lots of other factors.

For your example, leading an SRE team for a busy site is a responsibility, and it's useful to know - it seems kinda important. If you actually built the team, established their processes, led changes to some of the processes, implemented new tools, etc. that resulted in increased site reliability, that would be an accomplishment.