AU is the abbreviation for astronomical unit, a unit of linear measure based on the mean distance of Earth-That-Was to its Sun—some 93,954,524.4 miles—and a unit best used for intrasystem ranging. It is commonly superseded for use at interstellar and intragalactic scales by the more accommodating light year and parsec, and by kilo- and megaparsec at Local Group or other intergalactic distances.
Since about 62,399 AU make a light year, and 3.26ly a parsec, astronomers primarily use the former unit to measure the smaller distances between planets within a system (or between a star and its retinue), and use the latter two units for measuring the far larger distances among and between stars within a galaxy.