Here is some straw.
Here's a camel.
If you put a lot of straw on a camel, it would break his back, It would be the last straw that broke the camel's back.
Thankfully, this is just a metaphor for the final thing that pushes you over the limit of tolerance.
Like this - Marillion with thier song, The Last Straw. To me, it sounds just like Genesis- that's the last straw!