Clarification Needed please

Good morning,

I would like clarification on what < ESX 4.1 > under the Sumary tab > Resources the Memory Usage really stands for please.

Does that represent the used memory on the server based on all of the VM's running or what is it.

Here is a list of the servers on the machine at the time of the screen shot

Any help to clarify my confusion would be great.  I keep getting alerts of ussage in vCenter so just making sure I understand what is going on.

Thank you very much

Dana