What happens when you buy on a margin in the stock market

I don't think anything happens but when you're are buying on margin, it means that you're borrowing money from a broker to purchase stock. You can think of it as a loan from your brokerage. Margin trading allows you to buy more stock than you'd be able to normally. When this happens, it's known as a margin call.

the enslaved people onboard suffered from illness with some frequency

they made rome the dominant power in the mediterranean.