When you specify this option, PGPMenu will ask you which key you would like to see, and will display the key fingerprint of every key on your keyring that matches that string. It will pipe the output through the pager so you can easily view it.
If you specify an empty string, it will print the fingerprints of all the keys on your keyring.
When it finishes, PGPMenu will wait for you to tell it to return to the key management menu.