What does it mean when they ring the bell at Trader Joe's?

Excuse me, I couldn't help but overhear your conversation about Trader Joe's. Could you clarify for me, what exactly does it mean when they ring the bell there? I've noticed it a few times, but I've never quite understood the significance behind it. Is it a signal for closing time, a celebration of some sort, or perhaps just a quirky tradition? I'd love to know more.