Well... for example: some people know how to play a musical instrument, or juggle things, or speak another language, or do math, and some people dont. Everybody (presumably) can learn to do this things, which are controled by different parts of the brain.
This parts are there in everybodys brains. The difference is the ones who knows how to do this things are using this parts of the brain, while the ones who dont know are not.
I had some teachers that used to say that the brain is much like a muscle... if you exercise it, it becomes stronger and you can use it even more. But if you dont exercise it, it becomes weak and easily tired.
Being high is another example. Im sure when were high our brain is using different parts of it, parts that are not usually active when were sober, and thats why when were high we notice several things, we realize things, which we would never do if we were sober.
Also there are several people who has "psychic" abilities... and much of them were normal people who learned it, (there is courses of it), what shows that the ability to do it was already into their (and presumably everybodys) brains, just waiting to be used.
So, i think when they say we only use 15% of our brain they mean we only use 15% of our total possibilities, 15% of everything we would learn to do with our brains.
And if it were what they mean, i would think its a very high estimate... im sure our brains are FAR more powerful than that. Maybe this 15% refers only to the abilities they know, instead of all total possibilities...

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"Everything we do, everything we are, rests on our personal power. If we have enough of it, one word uttered to us might be sufficient to change the course of our lives. But if we don't have enough personal power, the most magnificent piece of wisdom can be revealed to us and that revelation won't make a damn bit of difference." Don Juan Matus - Tales of Power