StatsDawg

This represents the number of active Apache worker threads executing on the system, which is basically the number of simultaneous requests Apache is servicing. Though the behavior of this graph may differ slightly between the various Apache worker models, it does indicate how busy the Apache daemon is. This data is gathered directly from the Apache server-status page and, in particular, the "BusyWorkers" value. As a result, successive requests are not measured with this graph. This only measures requests being serviced at the same time.

In other words, this graph represents the number of people accessing your site at the exact same time, although this graph is best suited for tracking trends rather than short-lived traffic spikes due to the way in which the data is captured.