Sourdough discard is the portion of a starter removed when it is refreshed. It contributes flour, water and acidity to recipes such as crackers, pancakes and cakes. It may not have enough leavening activity to replace a ripe levain.
For a starter maintained at equal flour and water weights, half the discard weight is flour and half is water. Other starter hydrations require different calculations. Use discard from a healthy starter in recipes that cook it; discard does not mean spoiled starter.
Reference: Further reading.