So in a nutshell, what is Vplex?

Here is how I see what VPlex does. It sits between your arrays, and hosts, and  virtualizes the presentation of storage.  Instead of presenting storage to the host directly, you present it to VPlex. Then you go into VPlex, configure that storage and then present it to the host. Since, VPlex is controlling the storage, it can replicate the data to another VPlex. This allows hosts on another site to access storage as if it was local to them.

Is my understanding correct? But I don't understand how it replicate the data so fast in  VPlex Metro environment? There is going to be some lag between the two sites.