What does grand narrative or vast narrative mean?

A grand narrative is the idea that there is one narrative that stands above all others and encompasses them all. As a simple example, "Man should be good" is a grand narrative that looks down on literature, philosophy, and science and assigns "value" to them.

The three mentioned above, and indeed everything else in the world, should be in the service of some ultimate "good" or something (God?). This ultimate thing is the "grand narrative".

In the context of the above example, the Chinese Dream can also be considered half a grand narrative, which takes in everything that happens in China and assigns a value to it, based on the criterion of "whether or not it is conducive to the realization of the Chinese Dream"; and the Chinese Dream is in turn subsumed by a larger narrative (human dream? Human renaissance?) The Chinese dream is encompassed by a larger narrative (human dream? human renaissance?), and then it continues to be encompassed by the so-called grand narrative, which encompasses everything.

Today, in the so-called post-modern era, grand narratives are fragmented by post-structuralism, and thinkers like Derrida and Foucault are trying to reveal that there is no such thing as the "ultimate" thing in the world, and that all concepts are essentially equal, and that the ultimate things in common sense (God, goodness, structure, truth) are shaped by certain people for certain purposes. The common sense ultimate (God, goodness, structure, truth) have been molded by certain people for certain purposes, and do not have natural legitimacy. You can learn about post-structuralism if you want to understand this one.