Definition
The central difficulty or obstacle that complicates a situation or makes an achievement unlikely.
关键问题、矛盾所在
The plan looks great, but the rub is we don't have enough resources to execute it.
计划看起来很棒，但症结在于我们没有足够的资源来执行它。
Everyone agrees with the idea, but the rub is how to put it into practice.
每个人都同意这个想法，但症结在于如何将其付诸实践。
We know what the goal is, but the rub is how to overcome the technical challenges.
我们知道目标是什么，但症结在于如何克服技术挑战。