One meaning of the word "message" is an idea that a person tries to get other people to understand and believe. Politicians, authors, religious leaders, and business executives have "messages". You can talk about someone's message like this:

Her main message is that people need to incorporate movement into their daily lives, not just spend an hour at the gym every few days.

A "simple message" is one that is clear and easy to understand. This phrase sounds very positive.

Their message is simple: they want to reduce the size of the government.