The book starts at a point in time, and then gives two story lines. One back in time (Roman numerals) and one forward in time (Arabic numerals). Those two are alternating and equal, no one is superior to the other or just a "descriptive passage".
Once you understand it, the whole organization becomes very clear and no problem. If you don't know that, you'll struggle.
I love UoW, it's my favorite book (ahead of the superb Excession), but I haven't read all of the Culture novels.
Consider Phlebas is a terrible beginning (that I also endured), and the much-recommended Player of Games wasn't my cup of tea, either, though it's not bad.