Amp-Hours is a measure of the capacity of your battery – it tells you how much energy is stored in your battery.
Different racers/karts will use different amounts of energy to rocket their kart around the racetrack every moment, depending on factors like the overall weight of the kart or the mechanical losses in the kart design, etc. as you build a kart, you may care to take in to account how quickly your kart runs through the energy stored in the battery, so you can budget for how large of a battery will be necessary to make it through a sprint race (~15 minutes), and and endurance race. (75 minutes)
15 or 20 AH at the “48v” fusing class is typically plenty for a Sprint Race.
50AH or more will likely be required to run an entire Endurance race, without battery swaps.