It's just mint-flavoured sugar in an easy to carry form. The difference between brown and white is in the refining of the sugar (white sugar/brown sugar). It's a quick energy thing, which is why it's taken on expeditions. In fact you could only get it in hiking shops at one time, but it's now available in newsagents/sweet shops, even in chocolate-covered form. It's called cake in the same way you can refer to a cake of soap (or at least we can!).

I've got very few teeth left.