Kizumonogatari is the origin story of the Monogatari series essentially. In the original novels, it came directly after Bakemonogatari, the first series of stories, but is the first story chronologically.
The reason people are so excited for this isn't exactly because it's the origin story though, most of us know it already; it's because the movie's been in limbo since it was announced in 2011. After a couple of delays, Shaft eventually stopped updating everyone on anything that had to do with it. So, a real possibility after years of silence is really nice.