"The" is a definite article in English that is used to specify a particular noun. It tells us that the noun is known to the reader or listener, as opposed to being general or unspecified. For example, when you say "the dog," you are referring to a specific dog that both you and your listener know about, rather than any dog in general.
The word "the" comes from Old English "þē," which is related to similar words in other Germanic languages. Its usage has developed over centuries and has become a fundamental part of English grammar.