it's used in different contexts, but often in some form of publishing or another -- so in the case of CD production it would mean only one copy/master was made; it was not mass produced and distributed. Same thing with a book, magazine, brochure, print, painting, or just about any other media. Also used sometimes in web publishing -- where a big website might use a static template for all its pages (like this one), and populate them with different data, a one-off web page is custom made and "hard-coded".
http://www.worldwidewords.org/qa/qa-one4.htm http://dictionary.reference.com/search?q=one-off