The title Parable of the Sower is totally a reference to a certain parable that appears multiple times in the New Testament of the Bible. That story is—you got it—the parable of the sower.
At the end of the novel, Butler quotes the version of the parable told by Luke, which you can find in context here, though Luke's rendition is similar enough to Mark's, which we here at Shmoop have analyzed over hereabouts.
Long story short, the parable is about how a farmer spreads seeds, some of which die, but some of which grow. That farmer is a symbol for Jesus in the Bible, but it's a symbol for Lauren in this novel. Like Jesus, Lauren is spreading her radical new religious ideas, and sometimes they don't take hold (think of her conversations with Joanne Garfield or, to some extent, Bankole), but sometimes they do (think of her conversations with Travis, for instance).
So, it's a pretty straightforward title. With this title, Butler is hinting to us that what we're going to get is the story of a girl who's just starting to test out and spread her new religion. Will it work? Will Lauren be successful, like Jesus in the Bible was? Will the seeds grow? You'll have to check out the sequel to find out.