Trepanging is a way of catching sea cucumbers, which are kind of like squishy, worm-looking creatures that live in the ocean.
To catch them, people use something called a trepang, which is basically a long stick with a basket at the end. They stand in the shallow water and use the trepang to scrape up the sand and rocks on the ocean floor. When they feel a sea cucumber in the basket, they pull the stick up and take the sea cucumber out.
After they catch the sea cucumbers, they usually dry them out in the sun, which makes them smaller, lighter, and easier to transport. People have been doing this for a very long time, because sea cucumbers are used in lots of different ways. Some people eat them, while others use them for medicine or to make things like soup or jelly.