Atlas Subtitler consults some third party servers where the subtitles files (*.srt) are stored. For these logins some parameters can optionally be specified.
At the moment this is only possible for OpenSubtitles.org. Usually you will not have to enter something in the fields. But if you have an account on OpenSubtitles you can enter your personal credentials here. It is recommended to do that because you may get unlimited downloads or other advantages that the server grants to you.
You can use the Test button to check whether the login data works like this.
Just press the Documentation link to see the full informations about this.