The word "isometry" is a Greek-derived word meaning "equal measure". But let's not focus on the origins of this word, instead let's find out what it is and what we need it for.
Well, for the sake of briefness, isometry is a way of displaying three-dimensional objects in two dimensions. What does that mean to us? Basically, it means that we don't need to worry about perspective, or rather about what things would looks like in a true perspective. That's because an object in an isometric view is never distorted by either scaling it or shifting it, as it would normally be in a true perspective...
So, who can benefit from using isometry? Since Flash itself is not equipped with any 3D engine, game programmers can benefit from using it greatly. Actually many once-awesome games were made with the use of isometry (Diablo, Sim city, Age of Empires, etc.) in the times when 3D graphics were in its infancy.
Of course, programmers are not the only ones. Apart from them it is very handy to us: after all, why bother and draw in a true perspective, when isometry is out there? Just have a look at the room below.
Because the drawing was created in isometry, we can use it in a number of ways, like creating an RPG game. Also, it pretty easy to make all kinds of rearrangements :-)
I hope this helped you grasp the basic concept behind isometry. Now, let's move on and create an isometric object - that's were the real fun begins.