What is the golden rule of day trading?

Can you explain what the golden rule of day trading is, and why it's considered so crucial for success in this field? Is it a principle that applies universally, regardless of the market conditions or the trader's experience level? If so, how can a novice trader ensure they're adhering to this golden rule in their daily trading activities? Additionally, are there any common pitfalls or misconceptions that traders should be aware of when trying to apply this rule?