In grammar why a an the are articles?

In English, articles are Words that go before a noun (eg Apple) that tells you whether it is:

an - indefinite article, meaning it could mean any kind of Apple (an Apple in the basket)

the - definite article, meaning an Apple in particular (eg the Apple you're holding)

Note that "an" and "a" mean exactly the same, we use "an" when the noun begins with a vowel to make it more pronounceable ("a Apple" is more difficult to say than "an Apple")