This is an idiom taken from a boxing match. If you can imagine you're in the boxing ring being pummeled by your opponent, and they are beating the @%$# out of you. The bell sounds to end the round and the action ceases, so you've have been saved by the bell. And have a chance to recover!

Imagine that you are in a classroom and the teacher has just asked you a question that you have no idea how to answer, everyone is looking at you and you are trying to think of something to say to get out of the situation without looking dumb. Suddenly, the bell that signifies the end of class rings and everyone gets up to leave, you have just been "saved by the bell".

The expression comes from boxing originally. At the end of each round there is a bell to signify the end of the round. If someone gets knocked down towards the end of the round, they can be 'saved by the bell', and not have to face the 10 count from the referee.

Well, Thank you Michael, That really helped, can we use it as the same as in the boxing Ring ?