vBulletin Manual - Updating Counters

You will not need to use this particular vBulletin admin control panel area much during normal operations. Exceptions to this are after completing an import or after completing a mass move or prune of threads. Click on the Update counters... link (under the Import & Maintenance section) in the control panel. The following forms will appear:

From this screen, you can complete the following tasks:

Update users - Running this will update the user title and last post time for each user. You can choose the number of users to update on each cycle before starting. This process is not too intensive, so you can probably afford to have a fairly large value here.
Update Forums Info - This function updates the post count, thread count, last post time and last post username fields for every forum. It is useful after you have rearranged forums and their parents, or is you have moved lots of threads.
Update threads info - Running this will update the number of replies per thread, the original poster information, the last poster information and the attachment totals for each thread. This process is fairly server intensive, and you should not need to do it except after importing data from another board.
Build Search Index - This process rebuilds specified parts of the search index of the threads. If you make a change to the search indexing options, you will want to empty the search index, and then rebuild it from scratch. Please note that this is a very slow and server intensive process, and it should not be undertaken lightly.
Remove dupe threads - This option was created to tidy up after doing an import from another board. Often there will be duplicate threads created due to data corruption problems on the board which you have imported from. This process will go through and remove any duplicate threads that exist. It is quite server intensive, but just needs to be run once after importing.