Reason is the human capability of using facts and observations to make sense of the world. This is how humans are capable of understanding and explaining the physical rules by which the world works that we now call logic, science and mathematics.
In the distant past people believed that gods, demons, evil spirits, etc. controlled the world and caused things to happen, both good and bad. However,through thought and observation eventually realized that the world operates through rules and natural forces that can be understood.