Have you ever played with building blocks before? You know how sometimes you stack them up really high, and then they get pretty wobbly at the top? That's kind of like what happens in language with something called "phrase structures."
In a sentence, you have words that come together to make phrases, and then those phrases come together to make a sentence. Like building blocks! But sometimes, we move those blocks around to make the sentence sound different.
Here's an example sentence: "I ate a sandwich for lunch." If we want to emphasize the sandwich, we might say: "For lunch, I ate a SANDWICH." Did you notice how we moved the word "sandwich" to the end of the phrase "for lunch"? That's kind of like moving a block from the bottom of our tower of building blocks all the way to the top!
When we move words (or blocks) around like this, we call it "heavy NP shift." It's called "NP" because it usually involves moving around the "noun phrases" in a sentence (like "the sandwich" or "my cat"). And we call it "heavy" because it's like moving a really heavy block to the top of our tower of blocks.
So basically, heavy NP shift is when we move blocks (or words) around in a sentence to make it sound a little different, kind of like moving our building blocks around to create a different tower!