Canyoning? Im unfamiliar with that term.
Simply said, you drive to the top of a mountain, put on your gear (extra strong wetsuit, waterproof solid boots, rope and other gear) and you trail down the mountain in the river that has formed coming down from the mountain.
It's great fun as well quite dangerous, each year many canyoneers even experienced ones die because they slip and fall or it starts raining, the canyon fills itself up and the canyoneers simply drown.