Banks has an approach to exposition that could best be described as "Kill it with fire." His goal is to only ever provide you with exactly as much information as you need to make it through the next paragraph, and nothing more. Very often he deliberately leaves you confused about critical aspects of the world building in order to intrigue you with questions that will be answered later. This can be jarring, especially if you're used to lore-dump heavy SF/fantasy.

Consider Phlebas is a bit of a rough start. I found it to be more rewarding on a second read-through after finishing several other Culture novels. By all means keep reading Phlebas, it's very good, but it's ok to sit back and sort of let the book happen, if you get my drift. It'll become clearer as you read.