Here on Earth, we measure distance in simple units- inches, feet, yards, miles. But the measure of distance becomes much more complicated when you expand it to include the solar system, where distances between space bodies can be millions or billions of miles.
To simplify calculations when dealing with such large distances, scientists invented a more basic unit of measure- the AU (or astronomical unit). One AU is equal to the average distance of the Earth from the sun, or roughly 93 million miles. You can clearly see the advantage of such a system. For example- It’s much easier to say that Saturn is roughly 9.5 AU’s from the sun than it would be to write out all of those zeros.
In a field of study as complicated as astronomy, every little bit of abbreviation helps.