Tumble is Pragmatic Play's name for cascading reels. After a win, the winning symbols are removed, the remaining symbols tumble down, and new symbols fill the empty positions. Tumbles continue until no new wins form.
It is used heavily in Pragmatic's pay-anywhere games, where a minimum count of the same symbol anywhere on screen pays. In those games' free spins, multiplier symbols often land during a tumble sequence and are applied to its total win.
Tumble, Avalanche, rolling reels and cascades all describe the same mechanic under different brand names. A tumble sequence is one game round, settled as one result.