A Digital Rights Management removal tool disables the DRM feature of a digital product. DRM refers to a wide variety of anti-piracy measures, some of which are restricting the product to a certain device, limiting the number of times the user can install a software and preventing note-taking on ebooks.
Even though DRM measures attempt to defend the copyright of a product, it is not illegal to remove them from an ebook as long as the book is not distributed. Users can remove a DRM measure with the purpose of reading the book that they paid for on other devices.
As of 2015, there is no court ruling on DRM removal for movies or software. Therefore, while it's legal to remove DRM from books, a user may commit copyright infringement if he removes DRM from a movie, application or video game.