|Seiners are likely after mackerel in Provincetown Harbor in the early 1900s.
Imagine a very long rectangle of netting with floats attached along the length of one of the two long edges, so the net hangs in the water like a curtain.
The long bottom edge of the net is weighted down a bit by a series of metal rings running the length of the bottom edge, with a long rope, or "purse line" threaded through the rings. The net is set in a large circle, as you see in the postcard above, around a school of fish swimming near the surface. When the purse line is pulled the rings are drawn together like a drawstring purse, so the fish can't escape by "sounding" or diving down and swimming away. Then, as the net is drawn in, the circle gets ever smaller, bringing the fish alongside the boat to be scooped in. On a larger boat the whole net full of fish might be pulled into the boat.
This was a common way to catch "schoolers," which are any of a number of fish that swim together in schools, like herring, mackerel, some varieties of small tuna, and others. Provincetown had a sizable mackerel fleet, numbering into the dozens of larger mackerel boats in the late 1800s, and seining was an efficient way to catch this abundant species.