What CAN bus carries
A modern car's Controller Area Network moves small, constant messages between its computers: steering-wheel button presses, ignition state, reverse-gear engagement, door status, climate settings, parking-sensor activity. The factory head unit is one of the nodes listening. Replace it with an aftermarket unit that does not understand the dialect, and the car keeps talking to a radio that is no longer there — the visible symptom is dead steering-wheel controls, a reverse camera that never triggers, or a unit that never sleeps and drains the battery.
What a decoder does
A CAN bus decoder (or interface module) translates: it listens to the vehicle's messages and presents them to the new head unit in a form it understands — a button press becomes a volume command, reverse engagement becomes the signal to show the camera feed. Better integrations run both ways, feeding the car the signals it expects from the radio so warning chimes and displays behave.
When a swap needs one
Roughly: the newer and more integrated the car, the more likely a decoder is required. Cars where the factory screen also shows climate or vehicle settings almost always need considered integration. Older cars with a plain radio aperture often need none. Vehicle-specific head units are developed around one car's integration, which is a large part of their value; universal units rely on the decoder and harness specified for the target car.
What to confirm before ordering
For the exact vehicle and year: which functions the documented integration preserves (steering-wheel controls, factory camera, sensors, factory amplifier), which parts are included or required, and whether the configuration is set by dip-switch, software or a vehicle-specific harness. Get it in writing — “supports steering wheel control” without a per-vehicle statement is the single most common source of integration returns in this category. The support page explains how to request an exact answer for a specific car.