An apex is a point on the inside of a corner where you want your wheels to be touching to give you the best possible exit speed. It is also referred to as the clipping point of a corner.
There can be an early apex corner and a late apex corner. A late apex corner would be ideally suited to sharp hairpin corners or corners that have a long straight preceding. This allows the kart to see more available circuit on the entry to the corner which enables the driver to get onto the accelerator earlier. Drivers will need to be aware that they could be leaving an opportunity for the driver behind to overtake when driving a later apex corner.