What does "undoable" mean?
Both, actually. In the sense that something can be reverted to its prior state, the word is formed by joining undo and -able, literally something that is able to be undone. In the sense that something cannot be done, the word is formed by joining un- and doable, literally something that is not doable. While the two etymologies converge at un- + do + -able, the order in which the affixation occurs is the important thing.
Source: Wiktionary and personal experience