It means how much easier it is to fix any issue taking minimum time. These systems are so mature that when an issue appears, companies are confident that either their engineers can fix it or the external developer community can suggest solutions or these systems are already battle tested and most issues can be fixed with some effort. There are very less chance of any issue which is completely new to the internal or external developers which might consume time to get fixed. Also strong documentation and customer support can help you fix any critical issues quickly. As I said battle tested, many companies have already used MySQL and that itself is a testimony for its excellence. This is overall called operations trust that if any issue occurs, high probability is there to get it fixed quickly.