"the" is a definite article; a/an are indefinite articles (we don't know how many things a/an actually refer to, but theoretically it's one, but not necessarily).

Yep! "An apple" is any apple in the world, and "the apple" is a specific apple-- either there is only one apple, or it is the one being talked about. Usually you have to introduce with a/an before using the: "A ball bounced down. Then the ball bounced away." But if it's very obvious, you don't always have to: "I was having Doritos, and I ate everything in the bag." ("The bag" is clearly a Dorito bag and needn't be declared.) If you forget to declare, people will be confused: "The dog peed on my fence again." ("What dog? Were we talking about a dog?") But if you declare it more than once, it sounds like there are more than one object: "A van ran a red light, and then a van followed me home." ("There must be a lot of strange vans today.") Protip: you can use "it" in cases where you refer to the subject multiple times without adding more in between, skipping the "the" part altogether: "A purple, flying elephant did a dance, and then the purple, flying elephant flew away." vs. "A purple, flying elephant did a dance, and then it flew away." Both are correct, but one is significantly shorter! But be careful: "A robot punched a tree, and then it fell over." ("Which thing fell over? The robot or the tree?") So use "the" when there's more than one thing. Have fun!

Basically, the definite article is used when you want to say 'it', but there's more than one possibility that 'it' could refer to. Ex. A dog was sitting in a room. It was hungry. A dog and a cat were sitting in a room. The dog was hungry. 'The' just specifies what you're referring to. 'A/an' indicates that you're referring to something new. Replacing 'the' with 'a' changes the meaning. Ex: If you say 'the dog', the listener will think 'what dog?' and will be extremely confused if you haven't mentioned a dog yet. I said 'the listener' above because there is at least one listener for every conversation and I'm referring to that listener. If I had said 'a listener', then it means you're talking to a group of people and only one of them will think 'what dog?'. Ex: We say 'the definite article' because there is only one kind of definite article. 'A definite article' would mean that there are many different concepts called 'definite articles'.

In that example, you use "a/an" to introduce the thing for the first time. Of course, this only applies to countable nouns. The second time you mention the same thing, you use "the", to indicate that we already know which one you mean. If you use "a/an" again, it's a new introduction for a new thing.

Also, you use "a" if the next word begins with a consonant. "A consonant" you use "A" because "consonant" begins with a C. You use "an" if the next word begins with a vowel. "An apple" because "apple" begins with an A.