How does Affine Transform really work in Java?

I have been using Affine Transform to rotate a String in my java project, and I am not an experienced programmer yet, so it has taking me a long time to do a seemingly small task.. To rotate a string.

Now I have finally gotten it to work more or less as I had hoped, except it is not as precisely done as I want... yet.

Since it took a lot of trial and error and reading the description of the affine transform I am still not quite sure what it really does. What I think I know at the moment, is that I take a string, and define the center of the string (or the point which I want to rotate around), but where does matrices come into this? (Apparently I do not know that hehe)

Could anyone try and explain to me how affine transform works, in other words than the java doc? Maybe it can help me tweak my implementation and also, I just would really like to know :)

Thanks in advance.