It's often used to describe the point where someone can no longer tolerate a situation and decides to make a change or react in a way they wouldn't have before.
Here are some examples:
* "My boss constantly yells at me, but getting yelled at for being 5 minutes late was the last straw. I'm quitting."
* "He's always been irresponsible, but forgetting our anniversary was the last straw. I'm ending the relationship."
* "After years of struggling financially, losing their home was the last straw for the family."
The idiom is often used in conjunction with the phrase "that broke the camel's back," emphasizing the idea of a cumulative effect leading to a breaking point.