Hydrated lime is a builder's concrete additive and a soil amendment. The high calcium, low magnesium (non-dolomitic) type can also be used for making a home-made version of sodium hydroxide by following the methods outlined here.
The pure form of the chemical is known as calcium hydroxide (Ca(OH)2) or slaked lime. It's pH is a high 12.4. Aglime and hydrated lime are frequently confused, but are chemically completely different. Aglime is crushed limestone (calcium carbonate).
As a soil amendment, it is a cheap way to raise the pH of large amounts of overly acidic soil. In aquaponics and hydroponics, it can be used to safely raise adjust pH and add calcium to the closed systems, though some people would just add seashells to their system. There are several grades of hydrated lime for the garden, a high calcium grade, a medium calcium grade, and a dolomitic grade. These three are all varying degrees of magnesium added, with the high calcium having the least, and the dolomitic having the most.