What does the title The Rape of the Lock refer to?
The Rape of the Lock is a "mock epic" or as Pope calls it in the subtitle, an "Heroi-Comical Poem in Five Cantos." This poem was meant to poke fun at the aristocracy. This poem is a classic form of satire. (Pope wrote this at the request of his friend, John Caryll, who asked him to write something that two feuding families could laugh about together.)
In the poem, loosely based on real people (Robert, Lord Petre and Arabella Fermor), Belinda (representing Fermor) wakes up and goes to a party, ignoring advice Ariel (a sylph or spirit) has given to her in a dream. She plays cards at a palace with the Baron (representing Lord Petre) and others. During the game, unable to resist the temptation, the Baron cuts off a lock of Belinda's hair. Calling it the "rape" of the lock is an example of hyperbole (exaggeration). This goes along with the idea of this poem being a mock epic. It takes a very trivial event (cutting hair) and treats it like an epic event such as the adventure in The Odyssey: treating a haircut like it was the crime of the century. The idea of women cutting their hair was frowned upon in Pope's time, but it would still be an exaggeration to call it rape.
The poem uses a lot of tropes and devices particular to epics, but often in mocking ways: sprites/spirits, the suiting of armor (Belinda putting on makeup), games, and a banquet. What makes it even more exaggerated is the lofty speech and that the gods (sylphs and spirits) try to stop the rape/cutting. It is an exaggeration that the cutting of Belinda's hair is so reprehensible as to require gods to intervene. As the Baron attempts to cut the hair, the sylphs try and stop him:
The peer now spreads the glitt'ring forfex wide,
T' enclose the lock; now joins it, to divide.
Ev'n then, before the fatal engine closed,
A wretched Sylph too fondly interposed;
Fate urged the shears, and cut the Sylph in twain
(But airy substance soon unites again)
The meeting points the sacred hair dissever
From the fair head, forever and forever! (Canto 3, 147-54)