It is based on a British childrens book series The Borrowers. I was somewhat similar with the story as decades ago I saw the TV show done in the 90s starring Ian Holm.
So it's basically about little people who live in your house and steal stuff from you (they call it "borrowing"... yeah right).
As usual for Ghibli, it had very high quality of animation and settings and musical score. The first third or so was quite enjoyable, especially Arrietty's first borrowing trip with her dad into big people's world. It became somewhat more boring later on though when the novelty wore off.