This parable has a meaning similar to the parable of the weeds ( Matthew 13:24-30) that is its opposite in the chiastic structure of the discourse. However the focus here is on the judgment at the end of the kingdom rather than the mixed citizens of the kingdom. In both parables there are good and bad elements, believers and unbelievers. Jesus will separate these individuals at the end of His messianic (millennial) reign. They will all fall into one of two categories: the good (believers) or the bad (unbelievers). [source][source][source]
The Greek word for dragnet, sagene, occurs only here in the New Testament. It describes a large net fishermen drew to shore between two boats. Sometimes they tied one end to the shore and the other end to a boat. Then they would sweep an area of the lake with it, possible a half mile long, drawing as many fish as possible to the shore with it. [1] Then they would separate the fish that they could sell from those that they could not. [source][source][source]