My understanding is that postmodernism is taking what was built by modernism and tearing it down, showing how it doesn't work, and otherwise deconstructing it and examining it piece by piece to the point that it loses all meaning. It is disillusionment with the modern world, and admitting that it's all bullshit. In a word: nihilism.
Then, Metamodernism is taking those pieces and using them to construct things, just not the things you started with, and not using the pieces in the same way. And thereby adding meaning back into the thing, just not the same meaning as before. So you are still admitting that the original was bullshit, but instead of blowing everything up and leaving, you're constructing something that works for you, with full knowledge it is arbitrary. In a word: existentialism.