Whatever you encounter and experience, it is not solid, it is like a dream. There is a zen saying that the whole world is upside down because it is the complete opposite of the way that we generally consider the world to be.
We overlay everything we perceive with our own biasis and we regard that as reality. But other people can see the same things, even physical objects, differently, and we can shift our own perceptions about things over time.
This slogan tells us to take this world less seriously, to try not to hang on to what we experience and see here. When we become a witness to our own thoughts we can see that what happens is much less solid than we think.