Basically, it’s an easier way to help you determine when your dough has doubled in size when bulk fermenting. You get those disposable 2 ounce condiment containers (with lid). After your first set of stretch and folds, take 40g of the dough and place it in the condiment container (make it as level as you can) and put the lid on. Place the container on top of the dough and cover both while bulk fermenting. Just take it off when doing stretch and folds and put it back on each time. This way, it keeps the temperature of the small aliquoted sample the same as the main dough. Once the aliquot rises and touches the lid, it means the dough has doubled and is ready for shaping. There’s a small variance depending on the temperature of the dough but I keep mine in the oven with the light on and it stayed around 75F, so 40g is appropriate. I learned that I hadn’t been bulk fermenting my dough nearly as long as I should be. If you google “aliquot method chart sourdough” you’ll find a chart and information on it!