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Binding is how a widget shows live device data and sends commands back. Two ideas do all the work: the widget ID and field paths.

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The widget ID

Every widget has an ID. Firmware pushes values to that ID, and the widget updates. This is the core loop from How It Works: 
// Firmware (Arduino)
device.updateWidget(targetId, "tempGauge", 27.4);
A Gauge whose widget ID is tempGauge now reads 27.4. Set a widget’s ID in the General tab; reference that same ID from firmware.

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Field paths

When a device sends a structured payload, a widget reaches into it with a field path in the Data tab. Paths use dot-notation for nested objects and numeric indices for arrays. Given this payload: 
{ "sensors": { "cargo_temp": 6.2 }, "speed": 48, "gps": { "lat": 18.9 } }
Field pathResolves to
speed48
sensors.cargo_temp6.2
gps.lat18.9
The platform resolves what it can and ignores what it can’t — your payload shape is entirely up to you.

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Commands back to the device

Control widgets (Switch, Button, Slider, …) send commands in the Triggers tab. The firmware receives them through its command handler: 
device.setUserCommandHandler([](JsonObject &msg) {
  // act on the command
});

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Multi-device dashboards

Different widgets on the same canvas can target different devices — bind each widget to the device it represents. This is how a single dashboard monitors a whole set of devices.

The core loop

Values up, commands down.

Widget reference

What each widget binds to.