This article describes what MOBI files are, how to open them, and how to convert them to another document format so that they will work on your device.
How to Open Mobi Book File on iPad and iPhone – iPad Mobi Reader
A MOBI file is a Mobipocket eBook file. They are used to store digital books and are specifically designed for low-bandwidth mobile devices.
MOBI files support things like bookmarks, JavaScript, frames, and the addition of notes and corrections.
Free programs that can open MOBI files include Librum, Calibre, Stanza, Sumatra PDF, Mobi File Reader, FBReader, Okular, and Mobipocket Reader.