Reason or reasoning is a way of thinking that uses logic and facts. People can use reason to decide what is true or best. It is different from obeying tradition or emotions to decide what things are best or true. In common English, it is using your head instead of your heart.
Reason has been an important part of philosophy since Ancient Greece. Many philosophers have thought about what reason is, and how to think using reason (Logic). Also, they have thought if religious thinking and faith are ways of reasoning, what we can know if we only use reason (Epistemology), and if animals or computers can think using reason.