The Greek noun amphiblastron refers to a casting net — a circular net weighted at the edges, thrown by a fisherman to fall over fish in shallow water. It combines amphi- (around) + ballo (to throw).
The amphiblastron appears in Matthew 4:18 at the moment Jesus calls His first disciples. Jesus transformed their vocation: "Come, follow me, and I will send you out to fish for people." The net that gathered fish becomes the metaphor for the kingdom's gathering work. The gospel, cast wide into human society, brings in all kinds of people.