User enumeration is a way to fetch your website’s user data through malicious scripts. The hacker can fetch only the username details with this. Since knowing a username means 50% work done for the hacker in executing a brute-force. It is a serious security risk.
There are a few known tricks that can bare your user details in front of the hacker. You can learn these in this blog post here with the step-by-step process to fix it.