A plugin is JavaScript in a .pbw that runs on the phone, inside the Pebble
mobile app, and offers two things to other apps: sources, data that any
watchface or watchapp can read, and actions, operations such as turning a
light off or adding a task that any watchapp can trigger. A watchface that
shows the weather no longer needs its own weather API and key; it subscribes
to weather/location and whichever weather plugin the user has installed
answers. The Pebble mobile app exposes some of its own data as built-in
plugins, and the Index 01 assistant will call plugin actions as
tools.
Plugins are a preview. The API will change before plugins are released to all users, and the Pebble mobile app ships with them turned off. Do not publish apps that use or provide plugins to the appstore yet.
The documentation is maintained alongside the implementation and the example plugins in the Pebble mobile app repository, and is reproduced here.