Jesus is referring to the acceptance and different ways in accepting God's word. How some of us take it for face value while others hang onto the thought for a little while longer before letting it drift out of their lives. Others then harvest His word and allow it to grow in their lives.
When Jesus tells this parable, he is talking to a large crowd of ordinary people. Jesus took to a boat on the water so that the crowds could hear and see him as he preached through the parables.
Jesus is talking about receiving the Word of God. There may have been some discussion on the topic prior to Jesus introducing the parable - but it is not mentioned by Mark, likewise, there don't seem to have been any specific questions asked by certain people either to assist their learning or to try to trap Jesus, that have caused Jesus to tell the parable.
Most likely this is just one of the topics that Jesus really wanted to talk about and have heard, because it is obviously one of the most important aspects to Christianity.
Jesus uses this parable because it shows clearly that there are a lot of different ways in which the Word of God could be received, and it was not just a case of people either receiving and accepting it, or rejecting it.
So for this reason, there are many different ways of receiving the parable as well. The audience (the general crowd) is compared to the soil, with the seed being the Word of God sown by Jesus. The different types of soil are the different ways in which people can receive the good news.
The seed that falls by the way side...