POSSIBLE BREAK-IN ATTEMPT! 意味着什么？

Question

服务器日志总频繁出现
Aug 30 18:34:13 server sshd[12997]: Accepted password for xxx from xx.xx.xx.xx port 55023 ssh2
Aug 30 18:34:13 server sshd[13000]: pam_unix(sshd:session): session opened for user xxx by (uid=0)
Aug 30 18:34:13 server sshd[13000]: subsystem request for sftp
Aug 30 18:34:13 server sshd[13000]: pam_unix(sshd:session): session closed for user xxx
Aug 30 18:34:13 server sshd[13013]: Address xxx maps to server.xxx.com, but this does not map back to the address - POSSIBLE BREAK-IN ATTEMPT!

这意味着什么？