What is the 30 day rule for TFSA?

Could you please explain the 30-day rule for TFSA? I've heard it mentioned a few times, but I'm not quite sure what it means or how it applies to my investments. I understand that TFSA stands for Tax-Free Savings Account, but what's the significance of the 30-day rule? And what happens if I don't follow it? Is there a penalty or does it affect my ability to contribute to my TFSA in the future? Could you provide a real-world example or scenario to illustrate how this rule works? Thank you for your help!