Big Note is a very simple plug-in which adds a big note field to the Metadata panel. You can store any text in the field, and it can also be searched by smart collections.
Any information entered in the Big Note field is stored in the Lightroom catalogue. It is never written to the images’ XMP files and is not included in files exported from LR.
That’s because Lightroom doesn’t yet permit a plug-in to use its XMP-writing engine, and it’s one reason why I have big reservations about how custom field metadata is handled in LR2, LR3, Lr4, Lr5….
I have therefore left the metadata definition and panel unencrypted so anyone who can hack Lua can modify these files. For example, it wouldn’t be difficult to figure out how to add a second custom field to your catalogue.