Complex sentence: Once a player gets the first six balls into the target a seventh ball is added (called the "gold ball") to the game, the first player to get the six balls and the "gold ball" into the target wins the level.
Split into simpler sentences: Once a player gets the first six balls into the target, a seventh ball (called the "gold ball") is added to the game. The first player to get the six balls and the "gold ball" into the target wins the level.