Android - What is this "Vo LTE" notification bar icon?

VoLTE means Voice over LTE. The icon notifies you that you are now able to phone over LTE. This means, it's not necessary in order to make a phone call to switch back to 3G (You cannot phone over normal LTE).
In short it's new and better. As far as I can tell you could disable VoLTE altogether, but I recommend you to simply live with it.