The -t flag tests a configuration file without restarting the sshd daemon. This is helpful if you are ssh-ed into a server and need to change the sshd_config file before updating your configuration management scripts - an error may prevent your server from accepting your next ssh session, or kick you off the box.

The -f option allows you to specify a file. Excluding it tests the current configuration file (i.e ./etc/ssh/sshd_config).

$ sudo sshd -t -f /tmp/sshd_config

This command can also be used to test for installed ciphers when changing cryptographic options, as well as syntax errors.