Mark is different to Matthew and Luke in that there is little of Jesus’ teaching outside of parables. Jesus is less of a philosopher, more of an action man.
The symbol, a winged lion, depicts the resurrection.
The focus is on Jesus power in healing the sick, and in ultimately being resurrected from the dead. The lion was seen as a symbol of resurrection because in medieval times lions were thought to be born dead, to be brought to life by the breath of their father. C S Lewis used the analogy of the lions breath for the lion Aslan in the Chronicles of Narnia.
Jesus showed that God’s kingdom as broken out among us primarily by what he did. Jesus did, and can still do, wonderful things among us.