The centre of gravity is where the whole cargo weight concentrates. Pushed far towards one end of the container, the unit becomes unstable under the crane and overloads an axle when it rides on a truck.
The CTU Code gives two practical guides: never put more than 60 % of the weight in one half of the length (the 60/40 rule) and keep the centre of gravity within ±5 % of the length centre. The load planner works out where it falls from the pallets you place.