Since the leg moves forward while the shin and foot naturally swing upwards, the easiest application of this kick is that of directing one's energy upwards, perhaps considering it a "kick to the groin". However, one can deliver massive force forward with this kick as well, which is considered its main application by most instructors. Directed forward, this is actually one of the most powerful kicks in Taekwondo, and it is quite often used in exhibitions and board-breaking competitions where power is demonstrated. It is a kick distinct from the push kick (mireo chagi) in that the power should be delivered instantaneously.
In order to not injure ones toes while executing this kick, it is usually delivered through the front base of the foot (ap chook), if not with the flat upperside of the foot (bal deung). If performed with the bare foot then the ball of the foot is used on impact with the toes drawn up to prevent injury. To strike with ap chook one has to raise one's toes so that the their tips will not be the first contact point. Even when directed forward, this is not a kick where the first contact point should be the base of the heel, as is considered beneficial in some other martial arts having a similar kick. In Taekwondo, one would strike forward with the ankle extended, so that the upperside of the foot forms a straight line with the shin, and with the toes bent back (pointing up). In other words an "ap chook ap chagi". Having the foot in any other position when directing this kick strictly forward would be considered highly unorthodox, and is a common error among beginners. An example of one variation considered to be unorthodox is the front kick used by the Ninja. The knee is raised all the way to the chest with the lower leg parallel to the ground and pointing at the opponent. The kick is then forced straight forward striking with the entire bottom of the foot evenly. This kick is usually directed at either the chest or abdomen.
In addition to being a kick in itself, the front kick is an exercise used by many instructors to teach the principle of lifting ones knee before the rest of the kick commences, something which is considered important in Taekwondo, where it is somewhat literally translated from the Korean ap chagi (앞차기), (and many kicking arts with the notable exception of Capoeira). In competition fights (known as "sparring" or "kyorugi") this kick sees little actual use, except possibly as a component in an improvised kick which is perhaps intended as an "an chagi" or "naeryo chagi".
It is common to slightly bend the knee of the leg one is standing on when executing this kick, and pointing the foot one is standing on somewhat outwards. As in all Taekwondo kicks, one will also try to get ones "hip into the kick", resulting perhaps in a slight shift of weight forward. In any case, this is a linear kick, and as such one that one can get ones weight behind.
There exist countless variations of this kick, and it can be used along with other kicks without one having to put ones kicking foot down in between kicks. A very common variation is "ttwimyeo ap chagi", a flying front kick which can reach impressively high.
Some instructors refer to this kick as the "flash kick". This is in tune with the line of thought which seems prevalent in the various Taekwondo forms, where the ap chagi is used very extensively in combination with relatively short range hand strikes and blocks, mimicking situations in which it would have to be performed quite quickly.