The apex is the innermost point of a corner. Drivers aim to clip this point while turning. Hitting the apex allows a smoother exit. It helps carry more speed through the turn. Missing it often compromises acceleration.
For example, a driver touches the inside curb at the apex. He then opens the steering for a clean exit. Choosing the right apex improves lap time. Some corners have early or late apex strategies. Precision at this point is essential.