Avalanche is NetEnt's name for its version of cascading reels. Instead of spinning, symbols fall into place as blocks; winning symbols shatter and new ones drop to fill the gaps, giving another chance of a win at no extra stake.
The mechanic is best known from Gonzo's Quest, where an Avalanche multiplier rises with each consecutive win in a sequence and is higher during free falls, the game's free spins round.
Mechanically it is the same idea as Pragmatic Play's Tumble and other studios' cascades. The name is branding; the maths still runs to the game's published RTP, and the whole chain counts as one game round.