Selling your friends bike, but promising to buy it back in future. You get the cash up front, but if that bike goes up in value you lose money when you buy it back. Otherwise, if that bike goes down in value, you buy it back cheaper than when you sold, and profit the difference.
The common question is how can you sell something you don't own?
If other investors own stock on margin, then their broker is able to lend their shares to you to sell (short), and you will be obligated to buy back to cover in future. That broker's investor never knows their shares were lent out. If there isnt a big enough supply of shares to borrow for shorting, then you can't short. Usually there are plenty of shares to short. In theory, if the entire amount of outstanding shares of a company are held in non-margin cash accounts, then it can not be shorted. An investor holding shares in cash, can't have them borrowed. There is naked shorting that exists, but not suitable for 5 yr old intro.
If you are not out right shorting stock per above, you can buy put options or buy inverse ETFs to effectively bet against the market in a "short position", although you are technically "long" buying low and selling high to profit on market downturn. You call sell short call options to profit from downside, but option strategy is different depending if you are in the money, out of the money, and underlying stock positions.