AppBridge is a part of app-sdk npm package. You can import it from "@saleor/app-sdk/app-bridge".
The goal of the AppBridge module is to provide communication (a bridge) between Saleor Dashboard and App.
The whole communication is performed in the browser - it won't work in the Node context.
Docs
The most recent docs can be found in the app-sdk repository.
Events and Actions
AppBridge provides bi-directional communication between Saleor Dashboard and the App (in an iframe context)
Events
Events are sent from Dashboard, and App can listen to them.
With events, the App can react, e.g., to theme changes or receive a token with a handshake event.
You can find all events here.
Actions
Actions are sent from App to Dashboard. With actions dispatch, App can ask Dashboard to redirect or show a notification.
You can find all actions here.