Whaling is the work of hunting whales — going out in boats, killing them, and bringing them back to use. People did this for hundreds of years, mostly for the oil in a whale's body, which was burned in lamps before electricity, and also for the meat and the bones. So when you read about whaling in old books, it is a job, a whole industry, with whole towns living off it. Today the word almost always comes with an argument attached. Most countries stopped whaling, and there is an international agreement against it, so now when someone says "whaling" in the news, they usually mean a country that still does it, and the sentence around the word is usually about whether it should be allowed. A few communities also do a small amount of whaling as part of their traditional way of living, and people argue about whether that is a different case. Grammatically, note that "whaling" is not really the -ing form of a verb you will meet — nobody says "they are whaling" much in normal talk. It works as a noun ("whaling was banned") or sits in front of another noun ("a whaling ship", "the whaling industry", "a whaling town"). That last pattern is where you will see it most.