Not all the necessary information is clearly stated. We use our knowledge to make inferences about the meaning. We don't always listen to every word, but make inferences based on the 4 types of knowledge. Comprehension is not understanding what words mean, but is understanding what speakers mean. Not everything is clearly stated, and it is often necessary to figure out the real meaning.
Inferences are the core of comprehension.
Many people assume that the meaning is contained in the passage, and the listeners job is to get the meaning out of the message. The meaning is not in the passage, but is constructed by the listener. This happens because listeners vary. Different people have different knowledge and different ideas about the world. A person with more knowledge about something may understand more than a person with less knowledge.