Split a daily calorie target into protein, carb and fat grams.
Those are the standard energy densities: protein and carbohydrate each provide about 4 calories per gram, while fat provides about 9 calories per gram. Grams are back-calculated from your target calories at those rates.
Protein %, carb % and fat % are meant to divide your entire daily calorie target between them. If they don't total 100%, the split either leaves calories unaccounted for or double-counts them, so the calculator flags it and withholds the result.
Each preset just fills in a different protein/carb/fat percentage split — Balanced is 30/40/30, Low carb is 35/20/45, and High carb (endurance) is 20/55/25 — as a starting point you can still adjust.