It means that missions will be unlocked in a semi-random way, probably differing from the order proposed in this chapter. There's no real route here, requiring you to complete them in a strict order (aside from the few last missions) - often you will have at least two missions available at the same time, which don't really interact with each other in any way. Story missions, those which must be completed in order to push the story forward work in quite a peculiar way in XCOM 2. Completed missions can be checked from the menu located on the bottom of the screen.