This page is reached from Services → Webmail Server, then the Advanced tab.
Here you can set various options for the VPOP3 Webmail/settings service.
If you check this option, then you can specify the user's password in a URL to Webmail
For instance, if the VPOP3 server is on 192.168.1.1, then the Webmail address would normally be
http://192.168.1.1:5108
If the Allow Password in URL option is checked, then you can go to the address like:
http://192.168.1.1:5108?user=postmaster&password=admin
to go to the Webmail and automatically log in using the specified details
For a bit more security you could use:
http://192.168.1.1:5108?user=postmaster&md5pass=MD5-hash-of-password
In this case, you specify the MD5 hash of the password. That makes it more difficult to find the original password, but does not prevent a replay attack using the details.
Generally, we don't recommend using this option, because of the security implications, but in some cases it can be useful.