When you buy a share in a regular company, you know exactly what you are getting. If you buy Equinor, you get energy. If you buy Telenor, you get telecom. But what if you buy a share that gives you ownership in an insurance company, a battery manufacturer, an ice cream chain, and a railroad, all at once? Then you have bought a conglomerate.
The most famous example in the world is Warren Buffetts company, Berkshire Hathaway. In Norway, we have historically had Orkla, which at one point owned everything from pizza and chocolate to media houses and aluminum plants, before they decided to focus more strictly on consumer goods.
Managing a conglomerate is a great accomplishment. The top management (the parent company) rarely interferes in the daily operations of the smaller companies (the subsidiaries). Their only real job is to collect all the profits, and then decide which of the companies should receive money to grow further.
A very well known phenomenon in the stock market is the so-called conglomerate discount. This means that the value of the conglomerate itself is often lower than the sum of all the small companies if they had been sold separately. This happens because investors often prefer to choose their own industries, instead of letting a top executive force them to own a little bit of everything.