What does Mark 4:11 mean?"The kingdom of God" does not mean being saved and going to heaven, although that is part of it. It means any situation on heaven or earth that reflects God's power, sovereignty, and holiness. Jesus is willing to give the twelve and the other disciples special understanding of how God is working at that moment.
"Secret" is from the Greek root word musterion, from which we get "mystery." It refers to something hidden, and not readily available to the public. It is only discovered and understood through divine revelation; in this case, Jesus providing an explanation. Daniel and Joseph experienced the mystery when they deciphered prophetic dreams; Nebuchadnezzar and Pharaoh's dreams couldn't be interpreted through stars or a standard book of symbolism. Daniel (Daniel 2:18–19, 27–30, 47) and Joseph (Genesis 41:1–36) only understood dreams because God revealed what they meant. In our case, the secrets are revealed through the Bible which is a recording of several revelations.
It may seem unfair that the disciples receive the mystery where the others don't, but the parable of the sower explains why they have access to more information. They are the "good soil" that readily accepts the seed and nurtures it to germination, "yielding thirtyfold and sixtyfold and a hundredfold" (Mark 4:8). In other words, they choose to stay and ask for clarification. They are the people Jesus will describe in Mark 4:24–25 who bring a large "measure" because they want to understand many things. Jesus honors their heart by granting them their wishes. They stand in contrast to the Pharisees and Herodians who seek to destroy Jesus (Mark 3:6), the crowd who wants to use Him (Mark 3:7–10), and Jesus' brothers who wish to hide Him (Mark 3:21, 31).
Jesus says that the crowd receives everything in parables. This includes Jesus' actions as well as His verbal teaching—even Jesus physical miracles have a deeper meaning than what's seen on the surface. All of Jesus' ministry is hidden from those who don't dig deeper. This doesn't make God's truth hidden, or mystical, or even complicated—it just requires that a person care enough to actually pay attention and be willing to learn. Anyone can do that, if they have the desire. The biggest parable of all may be that He did not come to save the Jews from the Romans, but to save everyone from sin.