Long-range dependency means that some words in a sentence might impact other words that are far away from them. It's like having a chain of dominoes, where one domino is far away from the others, but when it falls, it affects all the other dominoes in the chain.
Let's say you want to understand the meaning of this sentence: "The cat, which was black, jumped over the fence." To understand it fully, you need to know that "cat" is the subject of the sentence, "black" describes the cat, "jumped" is an action, and "fence" is the object of the sentence. This information comes from the words that are far away from each other, but still connected.
This kind of dependency is important in many areas, like language learning, machine translation, and text summarization. It helps us to understand how sentences work and how we can improve the quality of the algorithms that process language.