The apex is the innermost point of your line, usually later than mid-corner. Learn how to find it, spot an early apex and gain speed on the corner exit.
The honest answer: The apex is the point where your kart comes closest to the inside of the corner. It usually sits not in the middle of the corner but a little later. Whoever takes it too early does not gain speed but loses it on the exit.
This sounds like geometry, but it is a decision. The corner shape does not fix the apex, you do: it determines when you brake, when you steer and when you get back on the throttle. An analysis of 60 years of racing-driver textbooks states that the optimal apex depends on the vehicle, the track profile and whether the corner is followed by a straight or more corners. A late apex costs some speed on entry but allows earlier throttle and gives you more road on the exit. The early apex is regarded there as a typical beginner error [1]. This comes from car racing (TRANSFER), but the logic in a kart is the same. There is no kart study on apex position, so the practice rules below carry no number.
Do not give up if you do not yet know the turn-in point as a topic of its own. It follows step by step here.
How tight is the corner, and what comes after it? If a long straight follows, a late apex pays off: you lose a little on entry but gain on the whole straight afterwards. If the next corner follows immediately, a slightly earlier apex can be better so you are positioned correctly for it [1]. That is a rule of thumb, not a law.
Find a fixed point on the inside of the corner: the start of a kerb, a colour change in the asphalt, a piece of rubber. You need a marker, just as with the braking point. The apex is a waypoint, not a goal in itself [1].
Look at the apex already while braking, then just before you turn in at the apex itself, and afterwards on to the corner exit. Turn your head along rather than moving only your eyes. In an experienced racing driver in a simulator, the gaze jumped shortly before turn-in to the tangent point, the spot on the inner edge where the corner bends for your eye (one driver only, car only, TRANSFER) [2]. In another simulator experiment, ordinary drivers followed this gaze pattern, while racing drivers looked more variably and turned their heads more [3]. Both are hints from car racing, not proof for karting.
Deliberately turn in later than it feels right. Motorsport driver books advise starting with a late apex and only then bringing the turn-in slightly earlier [1]. You pull the kart, so to speak, from behind towards the apex. At first it feels like braking because you go straight for longer, but it is the safer way.
The exit tells you whether the apex is right. If you have to steer hard again at the exit to avoid sliding wide, your apex was too early [1]. If your speed on the following straight improves, you may bring the turn-in point slightly earlier step by step. If the signs of an early apex return, you go one step back [1].
Your milestone: you hit the apex three laps in a row within about a hand's width (without running over the kerb), do not need to correct at the exit and can go progressively onto the throttle. The time window for this is a rule of thumb, not a study finding.
The late apex is the forgiving one: you have more road on the exit if something goes wrong. Build-up instead of shortcuts means a metre too late rather than too early.
In a pack several karts often drive the same line. Never squeeze into a gap at the apex that is not there. If a marshal shows a yellow flag, you reduce speed and do not overtake [4]. And note that on some tracks kerbs must not be driven on: the briefing tells you.
This guide explains and motivates, it does not replace supervised training. Whether your apex is truly right is seen faster by an instructor at the track or a video from behind than by you.
Plan the corner backwards: first the exit, then the apex, then the turn-in point. First ask where you want to be on the track at the exit so that you can get on the throttle early. The apex follows from that, the turn-in from the apex. Whoever thinks forwards usually plans the entry and pays on the exit. That is coaching practice. That the apex depends on what follows the corner is also in the motorsport literature [1].
The apex is the point on your line where your kart is closest to the inside of the corner. It is usually not in the middle but a little later. Exactly where is your decision: it determines when you brake, steer and get back on the throttle [1].
Often you take the apex too early. You are then too fast at the apex and pointing outwards, must correct and lose speed on the exit [1]. Start with a late apex and check whether you can get on the throttle earlier at the exit.
The racing line makes the corner as straight as possible: approach wide, pull to the inner apex, let the kart run wide again at the exit. The apex is its heart, but not always in the middle.
Understeer means the front axle pushes wide and the kart does not turn in. Oversteer means the rear steps out. An early apex can lead to sliding because you must tighten the line at the apex [1]. Smooth inputs are usually the fix.
Vision in the Kart: Look Where You Want to Go