Check out my very first instructional video on how to make rye starter from two malts.

What is malt exactly? Wikipedia states:

Malt is germinated cereal grain that has been dried in a process known as “malting”.

When grains (or most of seeds in general) are soaked in water, they start to germinate. This is the first stage of a growing process of a plant. Germination process develops enzymes, which start to break down complex saccharides like starches into simpler ones like sugar and glucose. The same is true for complex proteins present in the grains as well. Simpler variations of carbohydrates and proteins are then used as an energy source and building blocks for a new plant.

This process is halted midway to stop grains from growing into new plants and to get access to all of the enzymes which became available. These enzymes can then be used in multiple different ways for all kinds of products, including bread and beer.