You're seeing just the function listen_event, go back to Circuits.GPIO.Chip module for more information.
Link to this function

listen_event(chip, offset)

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listen_event(t() | String.t(), offset()) :: :ok

Listen to line events on the line offset

Circuits.GPIO.Chip.listen_event(mygpio_chip, 24)
# cause the offset to change value
{:circuits_cdev, 24, timestamp, new_value}

The timestamp will be in nanoseconds so as you do time calculations and conversions be sure to take that into account.

The new_value will be the value the offset value changed to either 1 or 0.