Host memory is a limited resource. VMware vSphere incorporates sophisticated mechanisms that maximize the use of available memory through page sharing, resource-allocation controls, and other memory management techniques. However, several of vSphere Memory Over-commitment Techniques only kick-in when the host is under memory pressure.
Active Guest Memory
Amount of memory that is actively used, as estimated by VMkernel based on recently touched memory pages so it is what the VMkernel believes is currently being actively used by the VM.