WordPress provides an XML-RPC interface via the xmlrpc.php script. XML-RPC is remote procedure calling using HTTP as the transport and XML as the encoding. An attacker can abuse this interface to brute force authentication credentials using API calls such as wp.getUsersBlogs.
It is possible to disable the XML-RPC script if you do not want to use it. Consult references for a WordPress plugin that does that. If you don't want to disable XML-RPC you can monitor for XML-RPC authentication failures with a Web Application Firewall like ModSecurity.