Force read (File=>Get e-mail)
This option was mainly introduced because of a "quirk" with Google mail Pop3 implementation. You should be able to leave this option blank for other accounts. You would really only need to check this option when you are using Google mail and you have many unread messages.
Usually, GSAK only reads the message headers so it can quickly determine if an email matches your selection criteria. When there are a large number of messages in a Google mail account, all messages must be physically read in order to "see" more messages.
For more information on this "quirk" see this thread