Once you turn DRS on and set it to fully automatic, unless you have some affinity / anti-affinity rules in place, your VMs will get up and dance around your cluster. In most cases this is fine and dandy. However, occasionally there are times where you will want to know where the VM was, where it went, and when it went there (sort of like tracking a teenager who is out for the night).
To do that, you can check DRS History in the vSphere client: