Malt is grain, such as barley or wheat, that has been soaked, germinated, and then dried in a process known as malting.
The malting process is conducted by soaking the selected grains in water until they germinate. Then the grains are transferred to the germination floor and dried with hot air to halt the germination process. Malt is critical for brewing because of the enzymes that develop during the germination and malting process. These enzymes are measured as diastatic power and are what enables starches to be converted into sugars during the beer’s mash process.
A beer with too low of a ratio of malted grains to non-malted grains in its grain bill will not have enough diastatic power to be able to convert all of the starches to sugars, and therefore will have a very low starting gravity and a low alcohol by volume.