Articles act as spotlights for nouns. They tell the listener whether you are talking about a specific, known item ('the apple') or just any item in a group ('an apple'). Without articles, your sentences will sound broken or confusing to native speakers.
Nouns are the names of things, places, or people. Articles sit in front of nouns to give them context. 'The' is definite—it points to something specific. 'A/An' is indefinite—it refers to one general thing. Sometimes, we use no article (zero article) for plural or uncountable things in general.
The biggest trap is using 'a' with plural or uncountable nouns (e.g., 'a waters' or 'a dogs' is wrong). Also, remember that 'an' is used before vowel *sounds*, not just vowel letters. For example, 'an hour' (h is silent) but 'a university' (u sounds like 'you').
Choose the context to see which article is the correct 'spotlight' for the noun.
Test your article instincts!
Fun fact: Many languages, like Japanese and Russian, do not have articles at all! That's why learning 'a' and 'the' is notoriously difficult for speakers of those languages. It requires learning to see the world through the 'definite/indefinite' lens.