It means that someone has tried to log in to your admin panel and he failed. What hackers basically do is they try to guess your admin login username and password.
We recommend changing your admin login page from default "/wp-login" to something custom. For example "/obvious-login-2000". It helps to avoid these kinds of hacking attempts because after you have changed your admin login page to some custom link then hackers scripts cannot find the place where to start guessing.
Also, read hints and news about website security from our security blog.