Here are a few. 'Lost keys. New start found.' This story implies that losing keys led to a new opportunity or a new chapter in someone's life. Another could be 'Met her. Heart still flutters.' which simply shows that meeting a particular person had a lasting impact on the narrator's emotions.
They are concise. For example, 'Bought dress. It had pockets.' Just six words but it's funny because having pockets in a dress is a small but delightful surprise.
Begin with a character or a situation. For instance, 'The child cried. No one noticed. This shows that you can quickly set up a scenario that implies a lot more than what's on the surface in just six words.
I'm not sure specifically which '20 best six - word stories' you refer to. There are many collections out there. Some might be about love like 'Met you. Loved you. Lost you.' Another could be a sad one 'Birthday. Alone. No calls. No cake.' And there could be a hopeful one such as 'Struggled. Fell. Got up. Kept going.'