Thunderbird - Building summary file?

[Bob Goodwin <bobgoodwin@xxxxxxxxxxxx>]

               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