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.
One For All definition according to MHA wiki
Long ago, a man with a Quirk that could steal and distribute quirks, forcibly gave his seemingly Quirkless little brother a Quirk or "Superpower" that allowed him to use stockpiled power in his body. However, the little brother already had a Quirk with no effect other than that it could transfer a person's Quirk to another. The little brother's Quirk and the Quirk given to him by his older brother merged and became One For All.
Also, All Might said that one for all is a union of two Quirks,
- A Quirk that stockpiles power
- A Quirk that could be transferred the quirk to others
As you mentioned in your question, Quirk Stockpile is the quirk that stock energy and physical power, people who have Quirk Stockpile don't have a physical limit they can increase their power by constantly working hard. Also, the first user couldn't beat the All For One, but All Might could do it. This means that, One For All quirk is passing the previous users' strength as well. That's why Izuku Midoriya has to work hard to make his body adapt to the power of One For All. The current physical body of Izuku Midoriya cannot use the 100% of One For All.
While what the answers before have said, the quirk stockpiles the fullest physical abilities of those who held the quirk previously. Though since it is phrased this way we can assume that this the reason All Might is able to react to everything moving at such high speeds is that the quirk also stacks reaction times. Any physical ability is stacked to the fullest ability of that person. Which in theory means it should be exponential. EX: Since All Might can take the full 100% his body is probably stronger than the average persons without it. And more so than the previous holder too. So as the quirk increases in strength, so does the base holders strength, speed, reaction time, hardiness. All that shit.
Though this might not be the extent of it. In the newer chapters we see that the previous holders may lay somewhere within the quirk, similar to the avatar being able to communicate to the past avatars. So there could be other things the quirk stacks as well. Willpower, Prediction, Muscle Memory, Ingenuity. Maybe Deku will be able to learn from and speak to the past holders. And maybe there is a secret avatar state we have yet to see ;P. I doubt that last one becoming real, and to be honest I would prefer that Deku be the best by himself. Though personally how I think Deku will become the best hero is that he will unite the heroes together (specifically his classmates and other great heroes) into an Avengers like team. And they together will become the new symbol of peace.
More new info regarding to what One For All can do coming from the manga, this has just revealed that,
One for All also transfers the quirks of former users to the current one, as speculated in the body itself of this question one year ago.
I believe it means that quirks from the past users can be stockpiled and used by the current holder of one for all, in the first chapter of the manga,( I'm not sure if its in the anima) when all might punches the liquid villain, its not that all might is just that strong, its a second quirk that is called " shock wave (page 31).
So when Deku was out of control once in the manga, these black energy strings stopped him. This is the result of the One For All stockpiling ability after the (5th, 4th, or 3rd I think) user of One For All had an ability called Black whip. When One For All is passed down to a user with a quirk and that user with a who already had a quirk passed it to somebody else without a quirk, the person who previously had no quirk now has One For All and the current users quirk. and then if the new user pass it to someone else, that person will also get the first persons quirk. If someone who already had a quirk gets One For All, that quirk gets embedded in One For All for the users to come.