If a player runs out of marbles, and is therefore unable to to drop one in at the top when required to do so, then that player has lost, and the game is over.
If, at the start of a player's turn, they have few marbles, and there is no way that they can use them to cause any to drop out at the bottom, then they may pass. It then becomes the next player's turn. If, however, there is a way that they can cause at least one marble to drop out at the bottom (other players may point out such a way), then they must play in the usual way.
Rules of Avalanche.
More about Avalanche.