Comprehensibility is not “easy to understand” itself; it is the noun for that quality — how easy or hard a thing is to understand. Teachers, lawyers, and researchers use it about texts, speech, exam questions, and signs: they ask whether the comprehensibility is high (clear) or low (foggy). It is a long, formal, classroom-and-paper word. In ordinary talk you would just say “Is this easy to understand?” Though actually the word does not praise the idea inside the text, only the way it is put. A sad or strange thought can still have high comprehensibility if the sentences are clear.