Comprehend is a service that utilizes NLP, Natural Language processing, it uses ML to find relationships in text. It can find key phrases, places, people, and topics. It can then determine if it is positive or negative, and even group the text files automatically into different categories that it defines based on frequency. Comprehend is incredibly useful for customer reviews where you need to be able to determine a positive or negative experience and even group together large amounts of similar reviews into categories so the issue can be found and addressed more easily.
Comprehend Medical is a similar but separate service that uses NLP to detect PHI, Personal Health Information, in unstructured clinical documents such as clinical documents, discharge papers, etc, and then takes key points from that data and uses ML to structure the data, highlight the important medical information.