"Shorting" a stock means selling it without owning it, essentially owning negative stock. You're betting that it will go down, and then when it does go down, you close your position by buying the stock (to get back to zero shares).

The risk in doing this is unlimited. If you short a stock at $100, and then it goes to $500, you've lost $400.

Your broker doesn't want to take liability for your loss if you're not able to cover it. So your broker will forcibly close your position for you.

Even if your broker doesn't close your position, you might do it yourself to stop losing money.

When a lot of people do this all at once, it's called a "short squeeze" as everyone who was short buys shares to close their short positions. It can make the price go up.

I don't follow Tesla's stock price, but Musk getting on Twitter saying something that could make the stock price go up could (and perhaps did) cause a short squeeze.