The OPF file is the file that contains all the "instructions" on how to make a Mobipocket book. It contains all the metadata, the list of files that make up the book, and what order they should be arranged in, the "Guide" section (which contains links to the cover image, the table of contents, and the point at which the book should be initially opened) and so on.
You can unpack a Mobi file to its OEB source (which will include the OPF file) using the "ebook-convert" tool that's a part of Calibre.
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