That affirmation is a popular myth which is widely believed
but without any scientific basis whatsoever.
And, moreover, it contravenes
all the known principles of neurobiology
and the theory of evolution.
It is frequently attributed to Albert Einstein,
using the argument from authority.
But there is no record of him saying that.
If that myth were true, 90% of brain lesions
would be innocuous.
Unfortunately, all brain lesions,
even, sometimes, very small ones,
have a much more severe effect
on the cognitive functions, motor performance,
or the level of consciousness.
The human brain represents only 2% or 3% of the weight of the body,
but it consumes 20% of the available oxygen
and 25% of the glucose
in the bloodstream,
so it makes no sense that natural selection
would have favoured
such a high percentage of inactive brain tissue
just waiting to be used.
It was probably the self-help industry
and pseudo-science that were most responsible for spreading the myth,
in an attempt to sell almost miraculous ways
to awaken our supposed hidden abilities
and so promise us that we can stand out
above other, ordinary mortals.