To oppose a majority opinion is an act of bravery.
Because of the title, Profiles in Courage, we know that Kennedy is highlighting senators who demonstrate courage, in his opinion. So, we can easily glean a definition from Kennedy's eight examples; here's what they all have in common: They all concern opposing a majority opinion.
Kennedy shows a version of courage defines by the willingness to do what one believes is right, instead of what will be expedient politically. The real world equivalent to this could simply be a person deciding to share an unpopular opinion, for justice's sake. A person brave enough to criticize fairly and generously when no one else is—that's Kennedy's favorite kind of courage.
The opinions of others are not the source of justice.
Ultimately, the point of this book is simply that because of group think, humans have a tendency to want to agree with the majority, especially when the majority is reacting to emotions that everyone shares. But, Kennedy's peak into history shows his passion for individual passion and belief.
This is a subtle commentary on bipartisan conflicts, and interestingly, he praises people for leaving their party (Adams), and for staying in a broken party (Benton), so the point is clearly that Kennedy believes people should obey their instincts, because different situations call for different ways of thinking.
The final word on this is Taft's criticism of the Nuremberg Trials. To oppose the prosecution of war criminals (Nazis, no less) made Taft look sympathetic to the Nazis, which he knew it would, but he still felt compelled to share his conviction, that the Nuremberg Trials were unjust.
Justice often means being flexible to accommodate opinions.
Although many politicians in this book are praised for speaking out against majority opinion, there are also instances of the inverse, where politicians look past their own personal opinions to help accommodate the differing opinions of others.
For this, take a look at the Benton passage. Thomas Hart Benton, Missouri, is praised for going against his own personal beliefs in order to prevent real bloodshed. His preference of peace above his personal agenda is an instance of bravery, because many painted him as a traitor for making the best of a bad situation.
Another instance of this is Sam Houston's flexibility during the Civil War. His temper bends until he believes that his actions will break his loyalty to the Union.
