A long squeeze is a strong, bearish price move that occurs when people sell stock. When a trader is long a stock, they profit when the stock price goes up, and suffer losses when the stock price goes down. As a stock trades down in price, people who own stock suffer trading losses.
To lessen the inherent risk of this, traders often times initiate stop loss orders at key price points commonly recognized as indicators of whether a stock is trending up or down: the 52-week low, 50 day MA, and 200 day MA.
In a long squeeze, the shorts have full control of the downward price moves and great market opportunity in weak stocks. Since someone who owns a stock, suffers trading losses in a stock that is trending down, a breach of these levels can trigger a powerful chain reaction of selling, causing a sharp drop in stock price up, called a "long squeeze".