It is a very popular saying. You might also hear that something is "the final straw".
Since I first heard this phrase when I was young, I always imagined a poor camel, walking through the desert, weighed down by straws....plastic drinking straws.
The meaning is that after many small problems or annoyances, even one more small one, when added to the others, will cause a person's temper, patience, etc. to finally break.
The image makes complete sense!
HOWEVER..... a CafeTalk student actually recently made me realize I was completely wrong!
The real image is a piece of straw or hay. A very large stack of small and light pieces of hay, and one more small and light piece will be too much.
But for some reason, I never made the connection!
Straw vs. Straw......
English is crazy sometimes!