Anything comprehensible is clear and intelligible — you can understand it. On the first day of school, if nothing your teacher says is comprehensible, you'll know you're in the wrong French class!
When you comprehend something, you understand it, and when things are comprehensible, they're clear enough for that understanding to result. Go down the hall to the French I classroom, and the teacher's slow speech and clear explanations will make the language comprehensible. If you write your homework out carefully and neatly, it will be comprehensible enough for your teacher to read it.