Why Is The Word “The” So Difficult To Define?

“It’s the most frequent word in the English language, accounting for around four percent of all the words we write or speak. It’s everywhere, all the time, so clearly it must be doing something important. Words have meaning. That’s fundamental, isn’t it? So what does ‘the,’ a word that seems to be supporting a significant portion of the entire weight of our language, mean? It must mean something, right?