the + [noun the listener knows]
[plural or uncountable noun] with no article, for things in general
The says "you know which one". It goes before one thing or many, and before words you can't count.
The thing was mentioned before:
There's only one:
It's clear from the place you're in:
The words after the noun say which one:
A plural or a word you can't count, with no article, means that thing in general – all of it, everywhere.
With the, it means a particular one or group.
Some common phrases about places and daily life have no article: at home, at work, at school, go to bed, by bus, have breakfast.
The sounds like "thuh" before a consonant sound and like "thee" before a vowel sound: the book, the apple.