Literally it means to be deceived. In other words, they know the truth about a person or a situation. There are many children who for years held their parents in highest esteem who later in life found out some dark hidden secret and in that moment they were disillusioned.
Marriage often brings disillusionment.
They have a very idealistic view of each other. After marriage, they soon become aware of faults and weaknesses they had not detected before. There are certain types of employment which can suddenly disillusion. For example, those who are in the field of law enforcement very soon become disillusioned about society in general. Those who are in the counseling field have occasion to know some of the more personal entanglements of people's lives and it has a very disillusioning affect.