Idle refers to the state in which there are no devices driving the bus. Occurs when the power supply on the driver side is turned off, or when a disconnection occurs.
The receiver output will be indeterminate, and this can lead to random data being received on the UART, causing spurious start bits, spurious interrupts, and framing errors. A countermeasure against this is a fail-safe against idle.

(Published in November 2021)