The Virtual Memory gauge in the Memory panel shows the total amount of swap space + RAM allocated to the machine you are diagnosing, and the amount of virtual memory currently in use.
If the total amount of virtual memory allocated to a UNIX host becomes full, the machine may halt all processes, or critical actions may be prevented from occurring. Running out of virtual memory may indicate a runaway process, or an under-configured machine.
The type of alarm that is activated is determined by the percentage of swap space currently in use.
See also: Swap Files alarm