What does Mark 4:2 mean?

"Parable" is from the Greek root word parabole. It means to set something similar alongside for comparison. Speaking "in parables" can refer to use of metaphorical stories, or it can mean the story is enigmatic and in riddles. Mark's emphasis is on the fact that Jesus' teaching is puzzling, more than the fact that it's in a particular literary form.

Mark does not record much of Jesus' teaching. Matthew 13 includes a few more parables which Jesus apparently taught at the same time. The parables in Mark 4 all elaborate on the purpose of parables as a teaching method. Parables are useful because they teach spiritual truths through stories about everyday life. Jesus, however, also uses them as a vetting process. Those who refuse to listen at all show their rejection of Jesus. Those who listen and then leave with only a surface understanding reveal that they are not interested in following Jesus. But those who listen and stay to ask Jesus the deeper meaning have softened hearts that are ready to hear Jesus' spiritual message.

Parables such as these work because God Himself established the objects these metaphors are based on. He also conditions us to listen to story through our need to understand history. Today, we are more likely to use allegory, like the Chronicles of Narnia. An allegory is similar to a parable, but the metaphor is based more on the character of people (like Aslan as Jesus) than more esoteric spiritual truths. Jesus' use of parables shows that fiction, when done properly, can be a very effective tool for spreading the gospel and deepening spiritual growth.