Looks like a brute force attempt after a port scan. I would not be overly concerned unless you have very weak passwords as they're very likely just randomly trying you because they can and not actively trying to specifically hack you, but do check out the firewalling section of the wiki and read up on securing your router to learn how to drop ssh (and winbox, telnet, etc.) from anything that couldn't possibly be a legitimate admin.
Specific answers require specific questions. When in doubt, post the output of "/ip address print detail", "/ip route print detail", "/interface print detail", "/ip firewall export", and an accurate network diagram.