The phrase "biggest threat" refers to something or someone that poses a significant danger, risk, or challenge. It is often used in reference to a situation where there are multiple potential threats, but one stands out as being more dangerous than all others. The term can be applied to various contexts such as national security, environmental issues, personal safety, and business risks, among many others. In essence, the phrase "biggest threat" highlights an issue or a person that poses the most significant danger in a given situation.