What does the expression “to reason” mean?

Since classical antiquity, and likely earlier, human beings have been conscious of the fact that they are in possession of a faculty that animals appear to wholly lack. That faculty, given the name ‘logos’ by the Greeks and ‘ratio’ by the Roman and Medieval thinkers, was translated into French as ‘raison,’ thus giving rise to our term ‘reason.’ Just what is this faculty? What are we doing when we engage in the activity of reasoning? Is it true to say that man is a rational animal, and if so, in what sense?