The Mobipocket file format is a binary format for the distribution of eBooks.
It is one implementation of the Open eBook Publication format with a number of proprietary extensions.
Mobipocket is an Amazon company since April 2005. This file is used to store metadata like bookmarks, annotations, last read position and other data. The MBP file is also used on the Kindle to store information about a book being read on the device.
There is a Perl script to extract notes from the MBP file.