Weight and Balance
Before every flight we have to confirm that the planned load is within the allowable centre of gravity range for takeoff and landing (as well as weight limits).
Balance is most critical during takeoff and landing because the speed is low (less control authority) and also that is when the most critical pitching manoeuvres occur. If the CG was too far aft it is very easy to overrotate and have a tail strike on takeoff, or over-flare in the landing. If the CG is too far forward the aircraft doesn't want to respond at all, which can be even worse.