James, CPU ready is one of the most important metrics in VMware monitoring, because it shows how multiple VMs are affecting each other.
CPU Ready = Percent of time there is work to be done for VMs, but no physical CPU available to do it on (all host CPUs are busy serving other VMs). One rule of thumb that I heard is that below 5% Ready is normal; anything between 5% and 10%, best keep an eye on the VM and the host. Over 10% (for extended periods) you should be planning on taking some action.
CPU Usage = raw, absolute amount of CPU used by corresponding VM at the given moment.