What most people already know, most items are quite breakable. Vials of holy water can be crushed. A book gets wet in a downpour and then swells to cartoonish proportions; swords rust; tents catch on fire. And pretty much everything wears out eventually. What an item save determines is if an item is destroyed through abuse.
For instance, a Warrior's sword might break if he swings wildly, misses his foe and instead strikes the blade on a rock. The sword could break, but then again, it might not. In this case, a save versus crushing blow would determine the outcome. (People have saving throws too, and those saves are listed in the "Injury and Healing" section.)
The types of saving throws for objects depend on what kind of object it is, and to what form of abuse it has been exposed. Thus, a book submerged in a river would warrant an item save of "paper versus water". If that river happens to be the heavily poisoned and ruthlessly caustic Dani Thun River, then the save would be "paper versus acid."
Just because an item makes its save does not mean it is wholly undamaged. In the aforementioned case, a book that passes its water save might be readable, but the GM could rule that the pages were curled and the bindings swollen. A sword that passes a save versus crushing blow is still fine for combat, but a small scratch or dent may mar its finish. Generally, items that pass a saving throw test usually suffer cosmetic damage but are otherwise fully usable. Items that fail a saving throw are generally destroyed or damaged badly enough to warrant repair.