I wrote a comment following an answer. A few minutes later, I see a horrible typo. So I edit my comment, click "Save Edits" but get a little red pop-up box saying "Comments may only be edited for 5 minutes". Why can't I save my correction? What does five minutes have to do with anything?
Comments are second-class citizens on the StackExchange network.
As such, they are only allowed to be edited in a short five minutes window. If your edit was submitted outside this window, you will see this red error message.
What can you do? If this is a fresh new comment, delete the old one and post a correct version. If the conversation already rolled onward and your comment received replies, it's best to leave a clarification.