Something understandable can be understood or grasped. It makes sense.
Since understanding something means you comprehend or get it, when things are understandable, they can be perceived or fathomed. If a friend cancels plans to see a movie because he's sick, that's understandable. Leaving one job for a better paying job is understandable. Eating a lot of cake on your birthday is understandable. Understandable things are easy to grasp and no one is likely to doubt or question them. The opposite of understandable would be weird, odd, or inexplicable.