Most of the time when punching in taekwondo, the fist is held horizontally at the moment of impact. In other words, the palm of the fist is facing the floor. A Vertical Punch (sewo jirugi) is a punch in which the fist is instead held vertically. I.e., the muscled base of the fist is facing the floor.
This punch is executed either with a single fist or both fists simultaneously. In the latter case, it is called a Twin Fist Vertical Punch. It is an effective technique when attacking a target or targets at a medium range. In ITF-style taekwondo especially, one normally keeps the elbow bent about 40 degrees at the moment of impact.