In this context it means you can take individual servers offline without taking your entire service down. So you can then update each server (even on production systems) live without requiring a maintenance window.

For bonus points you're also not babysitting manually provisioned servers but instead have your software installs automated. So any failure on a server or OS update isn't seen as a maintenance piece but rather just terminating the old server and letting your pipeline auto-build a new server. This is often referred to as "treating your servers as cattle rather than pets", though not everyone likes that analogy.