Johnson described Shakespeare's plays, and his tragedies in particular, as a "mirror of life." Because life is full of tragedy and comedy, and is at times tragicomic, Shakespeare's plays do too. As Johnson put it, Shakespeare "thinks only on men." Shakespeare recognized that in such locations as Classical Rome, medieval Scotland, and the royal court of Denmark, there were not just heroes and villains, but "buffoons," "drunkards," liars, and other characters. Many of Shakespeare's characters contain a combination of these traits within their personas, because, in short, they are human. Johnson's claim is that Shakespeare sought to portray the human condition as a complex, textured, contradictory situation.
It is important to understand why some of Shakespeare's critics did not like this aspect of his work. While many playwrights of the period (Johnson wrote in the eighteenth century) had modified Aristotle's classic theory of unities, they still expected that plays would not deviate from them too much. A play should have unity of place, action, and time, and its characters should clearly reveal moral lessons to the audience. According to this theory, there was no room for comedic scenes like the gravediggers in Hamlet.
Johnson believed that Shakespeare transcended this classic form of drama, and sought an authenticity that classical forms could not deliver. Mixing comedic scenes in a tragedy like Othello or Hamlet portrays life as it is, "good and evil, joy and sorrow, mingled with endless variety of proportion and innumerable modes of combination." In other words, Shakespeare's deviation from classical forms (which Johnson does not really think he knew about anyway) does not disqualify him as a great poet and playwright. It is actually what makes him great.