What does it mean if user name has invalid characters?

If a username has invalid characters, it means that it includes symbols, spaces, or letters that are not permitted by the system's username policies. Typically, usernames are restricted to alphanumeric characters and specific symbols (like underscores or dashes) to ensure compatibility and security. Using invalid characters can lead to errors during account creation or login processes. Users should check the guidelines for acceptable username formats to resolve the issue.