vBulletin Manual - Update Counters

The update counters section lets you ensure that counters such as number of replies in an issue or the number of issues in a project are correct. This is a maintenance section. You do not need to run either of the options here unless you suspect that the counters are incorrect. They should remain correct during day-to-day use.

There are two counters that can be rebuilt:

Rebuild Issue Counters - this will rebuild the reply, attachment, and petition counters in an issue. In addition, last post times and users will be checked.
Rebuild Project Counters - this will rebuild the total issue counters and last activity times for each issue type in a project. If you think you need to run this, it's generally recommended that you run Rebuild Issue Counters first to ensure that the issue data is correct.