If sourdough has a single defining ingredient, it's the starter — sometimes called the "mother." Everything else about sourdough baking depends on keeping one alive and active.
What a starter actually is
A starter is just flour and water, left to ferment until it captures wild yeast and lactic acid bacteria from the environment. Once established, it becomes a stable, living culture that a baker can maintain indefinitely by regularly discarding a portion and "feeding" it with fresh flour and water.
Why it's called "wild"
Unlike commercial yeast, which is a single cultivated strain, a sourdough starter hosts a whole community of wild yeast strains and bacteria, unique to the flour, water, and environment it developed in. That's part of why sourdough from different bakeries, or even different kitchens, can taste noticeably different even when the basic process is the same.
Keeping it alive
A healthy starter needs regular feeding to stay active — typically once a day at room temperature, or less often if it's kept refrigerated. Bakers watch for it to rise and develop bubbles after feeding as a sign it's strong enough to leaven a batch of dough.
Why it matters for the final product
The starter is doing the actual work of leavening and flavor development once it's mixed into a batch of dough. A well-maintained, active starter is one of the biggest factors in how a finished sourdough product turns out — which is why bakers tend to treat theirs with a surprising amount of care.