The word kihap is commonly used to refer to the "shout" that taekwondo practioners use when performing various techniques. For this reason, the word kihap is commonly translated as shout or yell. Sometimes it is translated as spirited yell, which is a little closer to the literal meaning.
The word ki ( 기 ) more closely translates as spiritual energy. It is similar to the Chinese term qi (also sometimes spelled "chi"). The word hap ( 합 ) translates as to gather and focus (or to synthesize, or to sum in total). So kihap means to gather and focus one's spiritual energy. The "shout" is considered to be the result of this focusing, not the cause. In other words, once you've focused your energy, the natural result is a shout.
Google Translate translates kihap as "fired up."
Some schools teach that a prolonged, loud shout is best. Other schools teach that the shout should be short, like a bark. In terms of what the shout sounds like, each person is generally free to make up his or her own shout. Some common examples:
In all cases, the shout should start in the diaphragm (rather than the throat or the mask) in order to provide power.
Sometimes, in competitive sparring, a competitor will make their kihap extra loud when they believe they have scored a point. The idea is that this might alert the officials to notice the point scored. Not all competitors do this however.