The article the is a determiner in a sentence when placed before a noun or a noun phrase. It is called a definite article as it points to a particular person or thing being mentioned in the sentence. It can be used before a singular or a plural noun, and before a countable and an uncountable noun.
We use the determiner the:
to refer to people or things which we know because they have already been mentioned.
to refer to a person or thing when there is only one.
before superlatives, words such as first, only, etc.
before adjectives so that the adjectives become nouns.
before a singular noun that is representative of a group of similar people or things.
before proper nouns.