What is the name of this Nintendo Switch accessory?
The accessory that ships with every Nintendo Switch is called the Joy-Con Grip. It's only purpose is to hold the Joy-Cons, in case you want to use them as a traditional controller. It does not charge the Joy-Cons at all. The Joy-Con Grip ships with every Nintendo Switch console (except the Switch Lite), and cannot be purchased separately.
The Joy-Con Charging Grip, meanwhile, is a separate purchase (at least for the launch Switch).
You can tell both grips apart by the fact that the Charging Grip is partially transparent: Picture taken from Polygon
This accessory is simply called Joy-Con Grip.
https://www.nintendo.com/switch/buy-now/accessories/