Description from extension meta
Connect Berkeley Snap! to Arduino boards
Image from store
Description from store
Snap4Arduino connector allows your Snap4Arduino online at http://snap4arduino.rocks/run to connect to your serial port and then, connect your Arduino device to Snap!
Running Chrome/Chromium with this plugin is the way to use Snap4Arduino without the desktop versions. So, it's the way to use it on Chromebooks.