Malt is germinated cereal grains that have been dried in a process known as "malting".
The grains are made to germinate by soaking in water, and are then halted from germinating further by drying with hot air. By malting grains, the enzymes required for modifying the grain's starches into sugars, including the monosaccharide glucose, the disaccharide maltose, the trisaccharide maltotriose, and higher sugars called maltodextrines are developed. It also develops other enzymes, such as proteases, which break down the proteins in the grain into forms that can be used by yeast.
Depending on when the malting process is stopped one gets a preferred starch enzyme ratio and partly converted starch into fermentable sugars. Malt also contains small amounts of other sugars, such as sucrose and fructose, which are not products of starch modification but were already in the grain. Further conversion to fermentable sugars is achieved during the mashing process.