A "reverse username search" allows you to find information about someone from a simple profile username.
Most people use the same or similar usernames when creating new accounts so it's easier to remember. This is great news for you as it can allow you to locate someone you may have lost touch with years ago. Or even allow you to determine if someone you met online is telling the truth about who they are. (You can make sure the information they have shared with you matches up with the information behind their profile.)