"About face" is an English idiom which means a sudden and complete change of one's plans, ideas and actions. It is also used to mean a complete turn of direction in a physical sense as in someone who is walking north, suddenly turns direction and starts walking south. Someone who has turned and faced another direction is said to have done a "complete about face." The origin of the expression is not certain but it probably came into common use based on its meaning in the military.
The expression is used in the English military. When the drill sergeant shouts "about face," then the soldiers know to turn on their heels and face the opposite direction. One of the earliest citations of the expression that has been found is in a book called Practical Observations on the Art of War written in 1711 by an English soldier called Major William Young. In it, Young explains the meaning of commands including that of "to the right about face." "About" means to face the opposite way. A synonym for the term is "about turn."
There is a similar expression in Italian and French. In Italian, "voltafaccia" is translated to mean "to turn" from the verb "voltare" and "face" from "faccia." The expression is used to describe a turnabout especially when referring to a reversal of opinion or policy. "Volta face" is the word in French.
An idiom is a phrase which has a different meaning to the normal meaning of the words which form it. "About", for example, as a preposition means of or connected with and as an adverb means near, nearly or nearby. "Face" in its most common usage means the front part of the head. When used as a verb, it means to look towards something or confront something.
Learning idioms can be very difficult for second language learners. The English language has thousands of such expressions and this is one of the aspects of it that makes it hard to learn. "About face," when considered logically, should mean "connected to the front part of the head." The usual rules of compositional semantics do not apply and while some idioms may be understood by considering the separate components, others make no sense whatsoever. The use of idioms with ease is the hallmark of a native speaker and so speakers of English as a second language are obliged to learn them if they wish to be proficient.