What file gets purged with Purge Audit Log?

When you use the Audit Log Purge Management on the Site --> Utilities page, what file does that purge?  Is it the access.log in the HTTPServer --> logs?  Just want to make sure what I am purging before I do.  If it is that file, that will be great because that file can grow very large.  Is there a performance hit when purging?

Thanks!