The phrase "big mistake" means a serious error or blunder that has been made, often resulting in negative consequences. It can refer to an action taken or decision made that turns out to be wrong and causes problems or setbacks. The term is used to describe something that was thought to be good idea but ended up being bad, leading to regret or disappointment. In essence, it's a mistake of significant magnitude that has serious implications for the person who made it or those affected by it.