Why must many acorns be soaked before people can eat them?

Many acorns must be soaked before they can be eaten to remove tannins, which are naturally occurring phenolic compounds found in the oak (Quercus) species.12

While the nuts of "sweet" white oak varieties can often be consumed directly, the nuts of many other species have a bitter taste due to these compounds. Tannins are astringent and serve as a deterrent to herbivores in nature. By grinding the acorns and soaking them in water, the bitter tannins are leached out, making the nuts palatable for use in foods such as mush or bread.12