Baker's percentages are a tool bakers use to communicate so that recipes can be scaled up or down. They appear as ratios used to express the amount of ingredients used. If a recipe states a dough is to be 70% hydration, like this recipe, that means for every 100 grams of flour, you add 70 grams of water. This extends to salt, yeast, and other additives.
For this recipe the important thing to note is the ratio of ingredients: 100 parts flour, 1.5 parts salt, 1 part instant yeast, and 70 parts water.