A starter dough is made by mixing yeast, flour and water then leaving it covered in a warm place to ferment. Once it ferments, becoming bubbly and smelling sour and yeasty, it is ready for use.
Restaurants usually make a larger portion, the remainder of which is constantly replenished by regular, scheduled additions of flour and water, so the dough is always ready when needed. Over time, the dough will become more stable and sour, adding flavour and texture to breads.
The first batch of breads that you make using new starter dough may not be as flavoursome as you would have hoped. But the longer the starter dough has been maintained, the better will be the flavour and texture of the breads that are made using it.