To be more exact it's gambling with other people's money. It's the equivalent of borrowing from the bank to put money on a dog. So long as the dog wins there's no problem paying the bank back. That's how it is on the stock exchange in a bull market. But just to make it interesting, all of a sudden all the dogs start to lose for no obvious reason.
All shares go down in what is called a bear market.
That is what happened in October 1929. Catastrophic The 'explanation' of margin trading doesn't explain the sudden reversal of trend. It helps to explain why the reversal was so catastrophic and became so general. It explains why brokers were found washed up in the Hudson river with a pocket of nothing but margin calls. Margin trading was gambling with other people's money.