What is the 2 day trading rule?

Excuse me, I'm a bit confused about this '2 Day Trading Rule'. Could you please explain what it is? I've heard it mentioned a few times in the context of cryptocurrency trading, but I'm not entirely sure what it entails. Is it a specific rule that traders must follow, or is it more of a guideline? And why is it important? Also, are there any exceptions to this rule? And if so, what are they? Thank you for your time and assistance.