Articles are used to indicate whether a noun is specific or non-specific.
"A" and "an" are indefinite articles used before singular countable nouns for the first time or when non-specific. (e.g., A cat is sleeping on the rug. I would like an apple.)
"The" is the definite article, used before specific nouns (the cat I saw earlier), things already mentioned (the apple you gave me), or unique things (the sun, the moon).
No article is used for uncountable nouns (milk, water), proper nouns (London), and plural nouns used in a general sense (People eat food).
Examples:
Incorrect: I saw a movie yesterday. (if it's a specific movie you already mentioned)
Correct: I saw the movie you recommended yesterday.