Thunderbird - Building summary file?

[Bob Goodwin]

                I'm using Thunderbird 1.5.0.2 and FC-5 with current updates.

                Periodically when I check mail it displays the message
                "Building Summary File for [Inbox, whatever I select]"
                and I have to wait for it to finish whatever it does. 
                After that I have to select the threaded display of
                messages again!  I don't know what this "summary file"
                is but I don't think I want or need it.

                Does anyone know what this is about?  I have only become
                aware of this recently.

                Bob Goodwin   Zuni, Virginia