The phrase "when you know, you know" is often used to express the idea that intuitive certainty or deep understanding about something, especially love or important decisions, comes without the need for explanation or rational proof. It means that once you truly understand or feel something, it's obvious and undeniable. This phrase is also notably used in song lyrics, such as in Lana Del Rey's song "Margaret," where it emphasizes the certainty and clarity of true love or important moments in life. The repetition in the song highlights the emotional resonance of recognizing when something important is genuinely felt or understood. In essence, "when you know, you know" means that some truths are evident only through experience and feeling, and when that moment comes, there is no doubt about it.