What do we mean when we say it today?
This phrase hasn't changed one bit since Shakespeare wrote it down. It still means exactly what Lady Macbeth meant. You can't change the past, so it's best not to dwell. There's no going back.
It usually has slightly more of a positive note when we say it today. Most of the time, people are saying that we should move forward and think about what to do next instead of focusing on what's already happened. But hey, it works when talking about murder, too.