I'm a bit confused, it's supposed to come out tomorrow but I don't know what it actually is?

There are threads asking this already - http://www.gamesas.com/topic/1331035-what-is-c0da/'s one for you (near the end of the thread).

A brief, more up to date explanation: it's a text by Michael Kirkbride, with illustrations by various artists, that explores and concludes various themes from the http://www.imperial-library.info/content/loveletter-fifth-era-true-purpose-tamriel, the http://www.uesp.net/wiki/Lore:The_36_Lessons_of_Vivec, and Morrowind; it's been a while in the works and we're all very excited.