The hardware acceleration functionality enables the ESXi host to integrate with compliant storage systems. The host can offload certain virtual machine and storage management operations to the storage systems. With the storage hardware assistance, your host performs these operations faster and consumes less CPU, memory, and storage fabric bandwidth.
Block storage devices, Fibre Channel and iSCSI, and NAS devices support the hardware acceleration.
For additional details, see the VMware knowledge base article at http://kb.vmware.com/kb/1021976.