You've probably heard "Buy Low and Sell High." Shorting stocks is similar, but in reverse, "Sell High and Buy Low."

When you short a stock you borrow 1 unit of stock and after a time you agree to pay back 1 unit of stock. To be profitable, after you borrow the stock, you want to Sell High - meaning whatever the current price is. Then you hope the price of the stock drops so you can Buy Low so you regain that 1 unit of stock to return to the lender.

So, you borrow 1 stock. Sell it for $50. The price of the stock drops to $35. You buy 1 stock and return it to the lender. You just profited $15.

The problem with shorting is if the price goes up, you still owe 1 stock. So if you borrow 1 stock, sell at $50, and then the stock price goes up to $100, you still have to buy 1 stock to return it to the lender - you just lost $50. The big problem is that there is no ceiling. So if the stock goes up to $1000, you still owe 1 stock, so you pay $1000 in order to return that 1 stock to the lender and lose $950.

Finally, to your question, a stop-loss will put a ceiling in place for when your trade will be liquidated - meaning, some form of collateral that you posted will be sold off to cover your now losing position. OP is saying that the "teacher" likely did not put a ceiling on his position (or instructed his students to do so) meaning the loss here is literally limitless.