(Observe as I reveal my ignorance of all things anatomical)
The collarbone is the arched bone which runs across the top of your chest, just under the base of your neck.
For some reason, it seems prone to breakage at the slightest provocation. Perhaps it's being named after a shirt component that causes all these fractures. My guess is that the other bones try to bump the collarbone into traffic and such. That's the sort of thing jealous bones would do.