Leaven is a natural raising agent, a fermented mixture of water, flour and the microscopic yeasts which are present in the air. It's a delicate living substance, sensitive to the external environment. The recipe is around 4000 years old and dates back to the Egypt of the Pharaohs, via a beautiful history...(note: the ancient Egyptians seem not to have had exclusive knowledge of the process: in Germany evidence of cooking a fermented dough has been found from 8000 BC (the Neolithic era).
The principle is extremely simple: water is mixed with a little flour. After a few days, using the yeasts in the air, the mixture "starts". Every 3 days it must be "refreshed", i.e. by adding an equal weight of water and flour. The mix froths and bubbles, smells rather like beer, sauerkraut, or vinegar. You can then use some to make leavened bread.