Oxford English Dictionary definition of a source - a place, person or thing from which something originates.
For those that have been indoctrinated with a belief system, it can be that the only source/beginning/start/cause for them to examine that belief system and hence bring about change to it, is their emotional reaction to it.
As an example of this I will speak of my own experience. I was taught the belief system of Jehovahs Witnesses from birth. For years I accepted it as the 'truth' and as being true. I remember studying with a man who had lost his mother and was desperate for answers. I felt uncomfortable with the answers I was giving. Those feelings of discomfort were the starting point of my questioning all that I had been taught and supposedly believed.
If I keep on doing what I have always done but I feel sad, angry, depressed about it I am not listening to those feelings; or the information those feelings are trying to give me (that something is not right in my world, that I need to examine my beliefs). Those feelings or emotions can be the starting point or source of my examining what is wrong in my world or my belief system.
Also if I smile at you what does it tell you about me? If I am crying and you walk into the room what does it tell you? If I look angry what does it tell you?
Your source of information is the emotion I am showing by my facial expression or body language. We read it every day in other people, we recognise that somebody is happy, sad or angry for example.
Emotion can be a source of information.