Complex sentence: Common-emitter amplifiers generally have a very high gain which can vary widely from one transistor to the next, as it is a strong function of both temperature and bias current, making the actual gain somewhat unpredictable.
Split into simpler sentences: Common-emitter amplifiers generally have a very high gain which can vary widely from one transistor to the next. The gain is a strong function of both temperature and bias current, and so the actual gain is somewhat unpredictable.