It's a word we use a thousand times a day, but trying to pin down its meaning turns out to be really quite hard. What does "the" mean? What is the difference between "the house", "a house" and just "house"?
To a native speaker of English, or another language that has the equivalent word (like French or German), it's so intuitive that it will seem like a silly question. But for speakers of languages that don't have it, like Russian, it's a mystery. It's very characteristic of native Russian speakers to speak and write without articles, like "Where is station?"
(There's an old joke about a Russian who arrives in England at Victoria station (I said it was old) and realises his watch has stopped. He approaches someone, who just happens to be a professor of philosophy, and asks, "Excuse me please, what is time?" To which the answer is, "That, my friend, is a very difficult question.")
A little thought shows that "the" somehow ties the noun in question to some mutually shared context. More than that depends entirely on the context in question. One of the harder questions in computational linguistics (and there are plenty of them) is to figure out the referent (what is being talked about) when "the" is used.
If I say to my partner "where is the car?" it is implicit that I means our shared car, or the car we happen to be using at the moment. But if I say "I saw a bad accident yesterday, with a car and a bus. The car rolled over", it means a car we've never talked about before, one I introduced in the previous sentence. I can even introduce a previously-unknown referent with "the". It's a common turn of phrase to say something like, "The car that parked in my space yesterday was back again today." It's really a shorthand for "There was a car that parked in my space yesterday, and it was back again today."
If I walk up to a stranger in the street and say "Where is the car?" they will just be puzzled, because they have no context to identify any specific car. But if I ask "Where is the station?", they will apply common sense to assume I mean the station in the town we happen to be in, or the nearest station.
"A", by contrast, implies something that is brand new to our discourse. "I bought a hat yesterday" means some hat that we've never spoken of before, at least when uttered all by itself. (It could be followed by, "Remember, the green one we saw last week", which changes the meaning completely).
Many, probably most, languages get along just fine without being able to make this distinction. Japanese has an interesting variation, the so-called "topic marker" (は, pronounced "wa"). This is generally considered to be very difficult for foreigners to get the hang of, yet if you think of it as "the" you will get along just fine. It is in opposition to the "subject marker" (が, pronounced "ga") which is somewhat equivalent to "a". If I say "car-wa has broken down", it means (in the absence of some other context) "my car" or "our car". But if I say "car-ga has broken down" it means "some other car, that we didn't know about before", maybe to explain a traffic jam - which particular car is unimportant. (Like everything in language it's more complicated than this, but it's a good enough explanation to get by with).
The thing that suddenly made me think about all this was something I wrote recently. There is another meaning of "the" where the referent is the notion of the thing, rather than the thing itself. If I say "The car has changed the way people live", it's obvious I'm not referring to any particular car, but rather the idea of the car, car-ness in general. We use this meaning without even realising it, yet it is completely different.
I wrote a blog article called "The Five Pound Note". It started out as a general discourse on British currency in general, and the banknote worth £5 in particular. But then I told the story of one £5 note in particular, that dropped from my mother's purse while we were boarding a bus when I was a child. In other words, I had switched the meaning of "the" in mid-article. This use of "the X" to mean (roughly) "some X which you'll find out about if you read this" is very common in literary titles: The Very Hungry Caterpillar, The Mouse That Roared, The Sting.
All of which is one small part of why computers have a long way to go yet before they can really figure out natural languages that humans use all the time without even thinking about what they are trying to say.