What does it mean when a girl lets you touch her boobs?

It depends. Did it happen intentionally?

If yes, then she's weirdly needy and emotionally unstable, and you should run the other direction -- unless you want a messy, clingy relationship with a chick who slings her boobs on top of men to get attention.

If no, then she's probably embarrassed, and you should be a gentleman, and pretend it didn't happen.