Complex sentence: The player has a time limit within which to capture at least 75% of the room's surface area, at which point the level is complete and the player progresses to a fresh room with one more atom bouncing in than in the previous level.
Split into simpler sentences: The player has a time limit within which to capture at least 75% of the room's surface area. When that percentage is exceeded the level is complete and the player progresses to a fresh room with one more atom bouncing in it than in the previous level.