What's a short?
Short-sellers borrow shares of an asset that they believe will drop in price in order to buy them after they fall. If they're right, they return the shares and pocket the difference between the price when they initiated the short and the actual sale price.

If they're wrong, they're forced to buy at a higher price and pay the difference between the price they set and its sale price.
Short sales have an expiration date, so when a stock unexpectedly rises in price, the short-sellers may have to act fast to limit their losses.

What's a Short Squeeze?
A short squeeze occurs when a stock jumps sharply higher, forcing traders who had bet that its price would fall, to buy it in order to forestall even greater losses. Their scramble to buy only adds to the upward pressure on the stock's price.

The flight of short-sellers and their impact on the stock's price is known as a short squeeze. Short sellers are being squeezed out of their positions, usually at a loss.

Short-sellers open positions on stocks t
they believe will decline in price. However sound their reasoning, it can be upended by positive news story, product announcement, or an earnings beat that excites buyers.
Or, a reddit group deciding to short squeeze the stock.

The turnaround in the stock’s fortunes may prove to be temporary. But if it's not, the short seller can face runaway losses as the expiration date on their positions approaches. They generally opt to sell out immediately even if it means taking a substantial loss.

Full article https://www.investopedia.com/terms/s/shortsqueeze.asp