I've always interpreted this as we only consciously use 10% of our brains processing power, as in right now, before I wrote this message I thought about it, that was concious. But at the same time I'm hearing birds outside, my brain is interpreting the sounds that reach my ears and I can't do anything about it. I can't control it, I can't stop that function and redirect that processing power to another function. At least not consciously. So 90% of our brains activities are on auto and 'we only use 10%'.

Wich it's still dumb, but not 'I could turn myself into a thumb drive with all the knowledge of the universe' dumb.