For something to be described as passable it must be clear of obstructions or able to be navigated. Another definition is for the object or person to be adequate or barely satisfactory. The adverb form makes the word passably, and the noun form is passability. Its opposite is impassable or not able to be passed.
Passible, on the other hand, is when something or someone is able to feel emotion or despair, especially in the sense that the object or person can gain this emotion from something else (i.e., impressionable). It has a noun form which is passibility. Something can also be nonpassible or unable to feel suffering.