Reason involves the ability to think, understand and draw conclusions in an abstract way, as in human thinking. The meaning of the word "reason" overlaps to a large extent with "rationality" and the adjective of reason in philosophical contexts is normally "rational", not "reasonable".
In action, reasoning involves a conscious attempt to discover what is true and what is best. Reasoning thought follows a chain of cause and effect, and the word "reason" can be a synonym for "cause".
As one way of coming to conclusions, reason is often contrasted to emotion, tradition and faith - the "rationalist" argument being that reason is the more reliable way to arrive at that truth.