Picture a heavy net lying flat on the bottom of a lake, with a boat pulling it slowly forward. The net does not choose. Whatever is in its path goes in: the fish that were wanted, and also weed, mud, shells and small creatures nobody was after. When the net comes up, the catch is there, but so is everything else. That one picture is the whole word. Move it onto dry land, keep the not-choosing, and you have a police operation that closes a whole area at once and checks every person in it.
From the word’s history
Simply drag plus net, a very old compound: Old English draegnet, with the same structure in Swedish dragnaet and Norwegian dragnett. The name is just the action it describes.
就是 drag（拖）加 net（網），而且是很老的組合：古英文有 dræġnet，瑞典文 dragnät、挪威文 dragnett 結構一樣。這個字的名字，本身就是它的動作。
Wiktionary etymology supplied in this packet (source_ref kaikki:3f6f69a190036ae4:1276411857): From drag + net, probably based on the model of Middle English draynet, Old English draegnet; cognate with Swedish dragnaet, Norwegian dragnett.
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