Short squeeze is a rapid increase in the price of a stock that occurs when there is a lack of supply and an excess of demand for the stock.
Short squeezes result when short sellers cover their positions on a stock. This can occur if the price has risen to a point where short sellers must make margin calls, or more loosely if short sellers simply decide to cut their losses and get out. This may happen in an automated manner for example if the short sellers had previously placed stop-loss orders with their brokers to prepare for this possibility. Since covering their positions involves buying shares, the short squeeze causes an ever further rise in the stock's price, which in turn may trigger additional margin calls and short covering.