Here Jesus compares the kingdom to the work of fishing, something with which His listeners would have been quite familiar. A typical "dragnet" was constructed with floats along the top and weights along the bottom so that when it was "cast into the sea" it would spread out horizontally and vertically.
This parable is similar to the parable of the tares in that good and evil exist together for a time. There are both good and bad "fish" that will find their way into the "dragnet" (i.e., God's kingdom). As the bad fish are physically removed from the good and disposed of, likewise the wicked will ultimately be removed from the presence of the righteous at the Judgment. It should be noted that separating the wicked from the righteous will not be done in a haphazard manner but with great care, as men would sort fish in broad daylight while sitting. No reference is made here concerning the fate of the righteous, but once again Jesus mentions the punishment that awaits the wicked.
As this section of text comes to a close, Jesus, like any good teacher, checks to make sure His students are understanding Him. "Jesus said to them, 'Have you understood all these things?' They said to Him, 'Yes, Lord.' Then He said to them, 'Therefore every scribe instructed concerning the kingdom of heaven is like a householder who brings out of his treasure things new and old'" (Matt. 13:51,52).
Although the word "scribe" technically refers to one who transcribes the law, it is also used as a common term for a teacher of the law as well (e.g., Ezra 7:6,10). Jesus' disciples would be scribes in this sense of the word. Since they affirmed that they understood the parables and the new teachings contained therein, Jesus instructed them to make use of that treasure at every opportunity. A good teacher should instruct his hearers out of both his past and present study and experiences. Jesus wanted His disciples to freely teach these new truths they had learned along with the old truths they had known for years.