This common phrase refers to a person who speaks for the rational, prudent action among people who are behaving irrationally. It is usually said in a context where a group of people are behaving or planning to behave in a destructive or foolish manner. The voice of reason opposes foolish behavior. It can also describe part of one’s own thinking:
The voice of reason in my head told me to proceed cautiously.
My college roommate was the voice of reason; she told us not to drink and drive.
Trump was not a voice of reason; he encouraged his followers to violence.
Because the voice of reason speaks for caution, some people may think of the voice of reason as a wet blanket—a boring person who tries to stop people from having fun. However, the phrase is not inherently negative.