Both comprehension and understanding can be used to talk about someone's ability to understand something. Understanding is a relation between the knower and an object of understanding. Comprehension usually used with of.
Queen Margrethe has called Mary and Frederik ‘a credit to Denmark’
Mental process of a person who comprehends; Knowledge about a subject, situation, etc. Understanding implies abilities and dispositions with respect to an object.
He noted bond's apparent lack of.
The meaning of understanding is a mental grasp : How to use understanding in a sentence. Or about how something works:
