这是驾驶室向机组成员或旅客发出的一种信号或提示，它在不同的情况下有不同内容。如果起飞前响，则是告知机上人员飞机即将起飞，请系好安全带；如起飞阶段完成后响铃，则告诉大家飞机已进入巡航状态，安全带可以解开，可以站起来活动等；如飞机在空中穿云层或遇阵风等，铃声则提醒旅客回座位，系好安全带，接近终点机场时的铃声则意味着飞机将要下降，请收好座椅和小桌板，系好安全带，机组人员做好相应准备等等，总之，它是一种友善的提醒。

There is a ringing before take-off or in the air, what does it mean?
The ringing is a signal or alert pilot sending to crewmember or passengers. There is different meaning in different situation: the ringing before take off means the flight will take off and seat belts must be fastened; the ringing during cruise means the airplane is in cruise condition and the seat belt can be removed. If the airplane goes through cloud or meets strong wind, the ringing means the passengers must return back to their seats and fasten the seat belt, when the flight is getting close to the target airport, the ringing means the flight is descending and the table must be stowed and the seatback must be in upright position and the seat belt must be fastened or crew members get ready, etc. Anyhow, the ringing is a friendly alert.