A percentage that shows the ratio of one ingredient to a base ingredient in the recipe. For buttercreams, I typically use butter as the base ingredient. For cakes, breads, and cookies, you'll often see flour as the base ingredient.

For example, we could say sugar is at 100% in a cookie recipe, which means that there is an equal mass of sugar and flour., i.e, 100 grams of flour and then 100 grams of sugar. And when you see baker's percentages, it's usually also an indication that recipes are given using mass measurements (grams, ounces, etc.) because that's the best way to annotate these ratios.

The usefulness of bakers percentages is that it gives us immediate context between recipes and allows us to make comparisons, e.g., this bread recipe uses 75% water but this one use 100% water. This informaion now gives us an idea that perhaps the 100% water recipe should feel more tacky during fermentation than the 75% one.