Symbolism, Imagery, Allegory
If you want to trace most of this book's conflict back to a single point, it's the moment when Bathsheba decides to play head-games by sending Farmer Boldwood an anonymous Valentine that asks him to marry her. She assumes that she'll never have to answer for the message, but Boldwood asks around and finds out that the note is in her handwriting. From that point on, he is obsessed with marrying her, even though she doesn't like him. It's a plotline that The Simpsons made good use of in an old episode where Lisa gives Ralph Wiggum a Valentine out of pity, and Ralph becomes obsessed like Boldwood.
When Boldwood makes his proposal, he makes a good point about how unfair it was for Bathsheba to lead him on, saying, "[But] I should not have spoken out had I not been led to hope" (19.17). This, of course, leads Bathsheba to smack her head and say, "I'm an idiot!" Actually, what she says is "The Valentine again! O that Valentine!" (19.18), because this is a Thomas Hardy novel and people start sentences with "O!"
Basically, she knows that it's her fault that Boldwood likes her, and this is the main reason why she has a hard time saying no to him for the rest of the book. In this sense, then, the Valentine tends to symbolize the kinds of problems caused by Bathsheba's little whims as well as the guilt she ends up feeling over what she's done.