When it was published in the First Folio edition of Shakespeare's plays, The Tempest was grouped together with the comedies. Although the comedy here is on the whole more subtle and understated than in, say, Much Ado About Nothing, it's there all the same, and it brings some much-needed comic relief to the proceedings.
One such example comes in the form of Caliban's shameless sucking-up to Stephano. Caliban is so desperate to come out from underneath Prospero's thumb that he's prepared to prostate himself at the feet of a drunken butler. In other words, Caliban wants to become the servant of a servant, and a drunken one at that. As part of his attempt to ingratiate himself with Stephano Caliban shamelessly offers to kiss his feet.
As with much of the humor in The Tempest, this is comedy tinged with more than a hint of sadness. For we can only feel pity for poor old Caliban: he's so anxious to escape Prospero's control that he chooses to degrade himself before such an apparently worthless nonentity.
Stephano provides additional humor, albeit of a much broader variety, earlier in the scene. In his drunken state, he mistakes Caliban and the jester Trinculo for a four-legged creature. The fool is hiding beneath Caliban's cloak to escape from a raging thunderstorm. Stephano observes that this strange creature appears to be able to talk out of its behind as well as its head:
Four legs and two voices—a most delicate monster. His forward voice now is to speak well of his friend. His backward voice is to utter foul speeches and to detract (Act II Scene ii).
This is low humor at its very lowest, with Stephano making a rather naughty joke about breaking wind.