Take a look at the example above of the sun. The word "Sun" is a noun. It is a noun for one single thing (sun, chair, car). This is called singular noun. Singular nouns should always have an "article" before it to provide us with more detail of the noun.
This is what "the" and "a/an" is. They are known as "articles," used before a singular noun. We can divide them into "definite" and "indefinite." (If you want to know the difference between "a" and "and," please refer to the previous lesson here.)
When we are talking about one noun in general, we use "a" or "an".
This chart below should help you
When you look at the sky, there are many many clouds but there is only one sun. So we use "the sun" because there is only one sun. It's always specific and clear which one it is!