The disciples did not understand the parable, so they asked. Before explaining it, Jesus asked a question. Barnes explains its logic: if they did not know "this which is so plain and obvious," how would they know "those which are more difficult and obscure"? Gill calls the parable "So easy to be understood, taken from things common."

Jesus' words "all parables" show that He treated this parable as a starting point for the rest. He was not turning them away. He had just told them, "Unto you it is given to know the mystery of the kingdom of God" (Mark 4:11). Matthew Henry draws the lesson: "if we understand not the plain truths of the gospel, how shall we learn those more difficult!"

No reply from the disciples is recorded. Jesus went straight on to explain the parable: "The sower soweth the word" (Mark 4:14). He described four kinds of hearers. By the way side, "Satan cometh immediately, and taketh away the word." On stony ground, they "endure but for a time." Among thorns, "the cares of this world, and the deceitfulness of riches, and the lusts of other things" choke the word. On good ground, they "hear the word, and receive it, and bring forth fruit" (Mark 4:15-20). Mark adds that when they were alone, "he expounded all things to his disciples" (Mark 4:34).

Those who asked were more than the twelve: "they that were about him with the twelve" (Mark 4:10). Barnes notes that when Jesus withdrew from the multitude, "a few followed him for the purpose of more instruction."

Two different Greek verbs stand behind the two uses of "know" in this verse. The first is oidate, and the second is gnōsesthe, which the BibleHub interlinear renders "will you understand." The first, in the perfect tense, looks at what they know now; the second, in the future tense, looks ahead to what they will come to understand.

This question shows Jesus as a patient teacher. He names the disciples' slowness, but He does not leave them in it. He explains the parable fully (Mark 4:14-20). Mark shows the same pattern later, when Jesus asks, "Are ye so without understanding also?" (Mark 7:18) and "How is it that ye do not understand?" (Mark 8:21), and keeps teaching them.

The parable itself is about hearing. The same seed falls on different ground, and the difference is in how the word is received. That is why Jesus goes on to say, "Take heed what ye hear" (Mark 4:24). Understanding the kingdom is a gift (Mark 4:11), and here Jesus gives it to those who came to Him and asked.