The phrase "saved by the bell" is commonly used to describe a situation where someone escapes trouble or avoids a difficult situation at the last moment. 

There are a few theories regarding its origin, but the most widely accepted one relates to boxing. In boxing, a match is divided into rounds, and a bell rings to signal the end of each round. If a boxer is in trouble and about to be knocked out, the ringing of the bell to end the round can save them from defeat, allowing them time to recover. 

This use of the phrase is well-documented and dates back to the 19th century. There is a popular but debunked myth that the phrase originated from a 17th- or 18th-century practice of tying a string to a bell that would be attached to a coffin. This was supposedly done to prevent premature burial; if a person was mistakenly buried alive, they could ring the bell to alert someone above ground. 

However, there is no solid historical evidence to support this origin. Another less likely theory is that the phrase comes from schools, where the ringing of a bell ends a class period, potentially "saving" a student from being called on or from an unpleasant situation in class. However, this is more of a modern association rather than the original source.