In this case, it is more or less what it sounds like. It is a large sheet that you spread out on the ground to use to keep you dry when the ground is wet. I do not know if all of them were literally made of rubber or not.
Nowadays, when you go camping, you have nylon and plastic and all sorts of other artificial stuff to keep you from getting wet. Back in those days, they did not have that kind of thing so they had to rely on these rubber blankets.
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