What does it mean that "One for All" allows to stockpile power? In My Hero Academia, they say that the quirk "One for All" allows the user to stockpile power. What does this mean exactly? Does it mean that the current user has the quirks of the previous users? Does it mean that the current user has the added strength, speed, resistance, etc. of the former users combined? Or does it mean he can save strength, speed, resistance, power, etc. when he isn't using it, having it "stockpiled", and using it altogether, at some moment? Or does it have any other meaning?
It would seem that all the unique users of One For All added their own power in the quirk- stockpile (One For All), as the earlier quirk user (his younger brother) couldn't defeat All For One, but All Might did.

It means that all the previous users had some powers of their own, which they passed down with the powers they had themselves received.