The picture would have been familiar to the commercial fisherman—then and now. The net in question is a seine, and in this time was drawn by two boats. The purpose is simple: scoop up all the fish in a given part of the waters, and sort them out on deck. In some ways the method is still used today.
The fact that two boats were used led many ancient writers to compare the dragnet to the Old and New Testaments. These are the handles; the net is composed of what is written therein.
Some of us are caught by the prophecies. How can we know that this Jesus is truly the Son of God? We can look back into the Old Testament and see the prophecies made many hundreds of years before his arrival, and how they were fulfilled by Jesus. Upon that faith, we can then look forward to his return, knowing what awaits the righteous and the evil.
Some of us care nothing for prophecy—but everything for forgiveness. Where else is forgiveness and cleansing found but in our Lord? Other religions proclaim a standard of righteousness, or tell us that guilt doesn't matter; this is the faith that meets the deepest need of the sinful soul—forgiveness.
Others are attracted by the wisdom and righteousness portrayed in the Scripture. When something is true, we often say it "holds water," or there are "no holes in it." The Scriptures are like that—a net with no holes, catching all.
Our Lord tells us the kingdom of God is like that net. We can see that in the church. There is no such thing as the "typical Christian." We are taken from almost every tribe, nation, land and tongue in the world. There are Christians who are rich, and of course many more who are poor. From the highest levels of society to the lowest, some will find the faith.
Interestingly, we also have our share of gar. Gar? Gar is a fish which—while it puts up a game fight—is not fit to eat. A fisherman with rod and reel hates gar, because they eat your bait and take up your time—for nothing. We have the same. Those who are inside the building but not in the church. Hypocrites, half believers, hangers-on—the list is long. But the time is coming when these will be separated from the true church. At the end of the age, our Lord promises, these will be cast out. So, if you're in church just because your school of fish swam in, remember: there's a net at the end.