Sourdough discard is the portion of sourdough starter you remove before each feeding. According to King Arthur Baking, you take some starter out to make room for the fresh flour and water; regular discarding keeps the yeast and bacteria in the starter healthy and stops it from growing into far more starter than you need.
You do not have to throw it away. Discard can go into recipes such as pancakes, waffles, crackers, cookies, banana bread, and pizza crust, where it mainly adds flavor rather than rise, and some bread recipes are written for unfed starter. Kept in the refrigerator, discard lasts several weeks; a dark liquid on top is normal, but pink or orange streaks mean it is likely contaminated and should be thrown out. If you do not use it, King Arthur suggests the trash or compost rather than pouring it down the drain.
Source: King Arthur Baking, “What is sourdough discard? And what should I do with it?”.
Image: Active sourdough starter, by spurekar. CC BY 2.0, via Wikimedia Commons.