The phrase "high alert" means a state of increased awareness, readiness and vigilance. It is often used to describe a heightened level of caution or preparedness, particularly in situations where there is a potential for danger or threat. The phrase can be used to describe a wide range of situations, from military operations to security measures at airports or other public places. In essence, "high alert" means being on the lookout for potential risks and taking necessary precautions to prevent or mitigate them.