For the most part, Shakespeare's Twelfth Night is an exploration of and exposure of human nature. In particular, Shakespeare exposes human nature and behavior as being sometimes pretty bawdy and foolish. The play is titled after the twelfth day after Christmas, which marks Epiphany. Epiphany celebrates the moment when the Magi, or Three Wise Men, arrived to give gifts to baby Jesus. While one would expect an Epiphany celebration to have a Christmas-like mood to it, apparently in Elizabethan times the holiday was a particularly rowdy and wild one. Shakespeare specifically wrote Twelfth Night for an Epiphany party held at one of the Inns of Court. This particular party was well-known to be "absolutely secular and even quite bawdy" ("Shakespeare's Twelfth Night"). The festivity was also knows as a "time of masques, revels, defiance of authority, and general foolishness" ("Shakespeare's Twelfth Night"). Since Shakespeare's characters perfectly portray the masquerades, revelry, and foolishness characteristic of that night, we can see that Shakespeare is using the play to spoof such behavior. Hence, one thing the reader can take away from the play is the understanding that human nature has a tendency to be rather foolish.
We see foolishness and bawdiness portrayed in Sir Toby's drunken behavior, in Sir Andrew's cowardly behavior, and in Maria's plan to humiliate Malvolio. We even see foolishness in Duke Orsino's obsession with Olivia, in Olivia's obsessive prolonged grief over her brother, and even in Olivia's rejection of Orsino, even though she knows him to be "virtuous," and "noble" (I.v.243). What's more, in the final song of the play, Feste sings about how foolishness is a part of what makes the world a harsh and dreary place, as we see in his lines:
When that I was a little tiny boy,
With a hey, ho, the wind and the rain,
A foolish thing was but a toy,
For the rain it raineth every day. (V.i.401-04)
The importance of these lines is Feste states that the young, or young minded like Maria and Sirs Toby and Andrew, treat foolish things like toys. Plus he links foolishness to the rain that falls every day, showing us that foolishness is linked to the harshness of the world. Hence, while Shakespeare's play makes light of foolish human nature, another thing the reader can take away from the play is that foolishness is a cause of trials and tribulations, just like the tribulation Malvolio suffered at the hands of other foolish characters.