FatMax marks the highest fat oxidation rate. Simplified, this is the maximum amount if energy (kcal) from fat combustion per hour. In endurance events, a high FatMax is associated with high endurance performance.
Whilst carbohydrate stores (glycogen) are limited, utilizing fat as a fuel can help to spare carbohydrates. FatMax is also a training intensity, which can be helpful to assign individual intensity zones for training.
CarbMax marks the intensity (speed or power) at which the combustion of carbohydrates reaches 90g per hour. This rate of carbohydrate utilization is the non maximum of carbohydrate absorption per hour (this is shown as less than 90 grams per hour).