In the army, “About face!” is an order: stop, turn all the way round, and look the other way. From that sharp half-circle, English took a second meaning that has little to do with feet. If someone does an about-face, they change their mind or their behaviour completely — not a little, not slowly, but as if they had spun to face the opposite way. A politician who fought a plan on Monday and praises it on Friday has done an about-face. You can use it as a noun (an about-face) or as a verb (to about-face). It is a bit informal and often a bit sharp: it hints that the person used to say the opposite quite loudly. Newspapers like it. Friends more often say “changed their mind” unless they want that sting. Though actually, people also use it for a real turn of the body, but the opinion-turn is the meaning you will meet far more in reading.