An SSH server can use the ‘keyboard-interactive’ protocol to present a series of arbitrary questions and answers. Sometimes this is used for ordinary passwords, but sometimes the server will use the same mechanism for something more complicated, such as a one-time password system.
Some of these systems can be automated. For this purpose, PuTTY allows you to provide a separate program to act as a ‘plugin’ which will take over the authentication and send answers to the questions on your behalf.
If you have been provided with a plugin of this type, you can configure it here, by entering a full command line in the ‘Plugin command to run’ box.
(If you want to write a plugin of this type, see appendix H for the full specification of how the plugin is expected to behave.)