The phrase "biggest hurdle" refers to an obstacle or challenge that is considered to be the most difficult, significant and daunting out of all other challenges. It's something that stands in the way of achieving a goal or completing a task, and requires extra effort, resources, creativity or determination to overcome it. The term can also refer to an obstacle that is particularly hard to surmount due to its nature, complexity or magnitude. In essence, "biggest hurdle" represents the most significant barrier standing between someone and their desired outcome.