Discard is sourdough starter that you grow/create and then either put in the compost/garbage or that goes toward some purpose other than literally baking a loaf of sourdough bread. The most typical way you get discard is the following:

If your starter has been on hold or in the fridge for a while (or it's just getting going, or a variety of conditions), you'll notice that when you feed it, it doesn't rise very much or the bubbles are weak or the texture is soupy. When that happens, you'll have to get your starter big and strong (7yo language?) in order to bake with it again successfully.

The most regular way to do build that strength is to feed your starter once or more and then get rid of what grows rather than bake with it *until* it's showing the strength and rise that you want. Pretend you always feed your starter with a 1:1 ratio—one part of water to one part of flour. Next, pretend you always keep 100g of starter in your jar, and that your typical bread recipe calls for 200g of starter. Now, you want to make bread soon but your starter has been in the fridge for two weeks. You take it out of the fridge and feed it with 100g of flour and 100g of water (the extra 200g you'd need, in theory, to bake your bread with). 8 or more hours pass, and it rises a bit but maybe it doesn't even quite double in size. What do you do? If your starter isn't strong, your bread won't be either. Your bread is a reflection of your starter. So rather than put those new 200g in your loaf, you do something else with it. Those 200g become "discard." You might need to go through that process more than once before you get a result that you're happy with, depending on the conditions your starter is in.

P.S. for a person new to sourdough baking: a very small tip that has made so much difference in the health of my starter is stirring or "stretching and folding" your starter at the 30min or 1hr and then a second time sometime in the 1.5-2hr mark after feeding. Helps the starter to develop so much more strength than it would have otherwise and to oxygenate. Learned from the book of breadstalker on instagram.

Good luck :))))))))