This log file, normally at INSTANCE_HOME/log/Z2.log, shows all requests completed by Zope. Its format is similar to apache's access.log. This one tells you what your zope server is doing.
Note that requests are written to Z2.log only after Zope has completed processing and sent an HTTP response to the client. If you are troubleshooting a slow or hanging server, you may wish to enable the trace.log which gives more time detail.