A trawler is a fishing boat that catches fish by dragging a big bag-shaped net, called a trawl, slowly through the water behind it. That dragging is what makes it a trawler rather than any other fishing boat — boats that use lines, or that sit and wait with nets, get other names. Trawlers range from small family boats working near the coast to huge factory ships that stay at sea for weeks and freeze the catch on board. Because of that, the word often turns up in news about fish stocks and sea damage, sometimes with a sad or angry tone, since a heavy net dragged along the seabed destroys a lot. You will also see the word in stories about the sea and about war — old trawlers were often taken by navies to sweep mines, because they were already built to drag things.