Chewbacca's only clothing is a large belt that he wears on his shoulder. On this belt, there are some hard shell boxes that alternate small and large.
What is in those boxes? Ammo? Snacks? And why are there 2 sizes of boxes?
The strap is his bandoleer (more commonly spelled "bandolier" and often just called a "utility belt"). I don't recall him ever specifically utilizing the contents in any of the books, but they typically contain items such as ammo, grenades, supplies or small tools.
In Chewbacca's case, it is most likely that his contains ammo and tools.
Unofficially, Peter Mayhew (the man behind the mask) explains on his own website:
Well, it couldn't be amno, because they let him carry it even when they're captured by the imperial troopers in Empire Strikes Back.
So I guess it's snacks.