Something that's questionable is uncertain or hard to believe. If a politician says things you doubt are true, you might describe his words as questionable.
You might say that it's questionable whether you'll reach your goal of getting straight A's in school, or secretly believe that everything your history teacher says is slightly questionable. In either case, you're highly doubtful. You can even describe a person as questionable, if he or she doesn't strike you as completely honest. The original meaning of the word was "that may be interrogated," from the Latin root word quaestionem, "a seeking, inquiry, questioning, or examination."