BdBrowser is an extension for your browser to load BetterDiscord. It's basically a simple NodeJS environment which should allow BD Plugins to run as in the client.
The installation is currently done by downloading this repository as ZIP file and loading it in your browser while having developer enabled.
Go your your chrome://extensions page and click this button and browse to the path where you downloaded the ZIP file into.
Now, that you loaded the extension you reload the tab where Discord is opened and BD should show it's changelog immediately.
You can install plugins/themes by pressing the Open [...] Folder button in the plugins/themes category of bd's settings.