Baker’s Math is a system that was setup to easily share bread formulas. You can begin to use it once you are using a scale to measure all of your ingredients. Here are some of the important aspects of it:
Total Flour Weight: The total weight of flour used in the formula. Count all types of flour used, and any flour that is part of a pre-dough or your starter.
Ingredient Weight: The weight of any particular ingredient. If an ingredient is used in a pre-dough, make sure you count it (most often with Water Weight).
Ingredient Percentage: The Ingredient Weight divided by the Total Flour Weight. The Flour Percentage will always equal 100%, but in some formulas you be split between several flours, such as have 60% Whole Wheat Flour Percentage and 40% Bread Flour Percentage.
Hydration: This is the Ingredient Percentage of Water. You also count any milk or juice, and 75% of any egg weight (because 3/4 of the egg is water). Do not count any oil or fat.
Blarf Factor: This is the Total Dough Weight divided by the Total Starter Weight. It is important because it lets you know how much bread you will end up depending on how much of your starter you use up.
Here is another good article explaining Baker’s Percentages and Hydration.