Host memory usages is the amount of RAM that has is allocated from the host memory for the VM to use, guest memory usages is the amount of memory that the VM is actively using at a given point.
When RAM is not contended on a system you will see the host memory usage close to that of the allocated RAM for a VM, as it is all allocated on VM boot, and then ESX has not bothered to reclaim it as it does not need the memory for anything else - so there is no need for a reclaim operation and the memory is left assigned to the VM for better performance. What you'll see as you start to get closer to the limits of the available memory in your system is the host memory usage coming down more into line with the guest memory usages - but on the whole before getting to this point if there is a massive difference between the host memory and guest memory usage for a VM it would be worth considering dropping the amount of RAM allocated to a VM, as reclaiming memory from a VM has a performance impact.
