Malting is the process by which Malt is produced. Grains are soaked in water over externded periods, during which they are drained, soaked, drained, soaked and so on. This causes the grain to germinate, or start to grow. Once germinated, the grain rootlet forms at one end, and tunnels beneath the grain husk to the other end of the grain. During this time, changes take place within the grain itself. Certain enzymes are formed which under normal circumstances would help the developing plant turn the starch in the grain seed into food for itself. The maltster will halt this "modification" by kilning the grain at certain temperatures (according to what type of malt is desired). Later, the brewmaster will again soak the grains in a process called "mashing". This allows the brewmaster to re-activate the enzymes, and use them to turn the starches in his mash into sugars for yeast food.
The process through which barley is transformed into malt, by artificially starting up its germination process, which will eventually be stopped at the kilning stage. See also the illustrated description of malting in the pages covering the making of whisky.