On the left side of each row, you will see an arrow displaying a multiplier. Every time a winning combination lands on that row, the multiplier will increase by one. More than one multiplier can increase at a time as long as any symbol in the winning combination is on that reel. If a lightning bolt symbol lands on that row, the multiplier will become active and be attached to any winning combination with one or more symbols on the row. There are also multiplier boost symbols that can raise the multiplier higher.
Look out for the black skull symbol. If that lands on a reel, it will deactivate the multiplier, and you’ll need another lightning bolt to reactivate it. Whether the multiplier is active or not, it will continue to grow when winning combinations or special symbols land on it.