The apex is the point of a corner on the inside of the track that allows the driver to make the largest radius. The larger the radius, the more speed can be carried through the corner.
When discussing the racing line and the fastest way around the track you can move the clipping point or apex to improve lap times. You may hear the terms late / early apex. A late apex moves the clipping point further around the corner while an early apex has the clipping point sooner than a traditional apex.