Toning or unstable shoes are a segment of the athletic market in which the shoes are deliberately made to be unstable so that walking in them is harder. This is supposed to increase the workout effect of the shoes. The evidence that they actuall do that is very mixed.
There have been a number of class action suits and settlements with the Federal Trade Commission due to the lack of evidence that the claims that were being made for the shoes.
Despite the lack of evidence for the toning claims, the shoe do have thier uses to manage conditions like hallux rigidus and some post postural problems.