When you learn English, it is utterly essential to learn about articles. Those are the words a, an, and the.
To native speakers, these are easy but to most learners of English, they are actually really hard. Think about it: Why do we say "I can play football" but "I can play the guitar"?
It's weird, right?
The Definite Article
We use the definite article (the) when the speaker and listener both know something. For example, there are many moons in the universe but we say "the moon" because everyone knows which moon we are talking about.
We can also talk about things that are just obvious to the speaker and listener, such as "Please hand me the cup." In this case, the speaker and listener both know which cup is being discussed.
The Indefinite Article
We typically use these when there are several of something and we refer to one of them. For example, I said, "Please hand me the cup" because both the speaker and listener knew which cup. However, we could say "Hand me a cup." This means any cup.
This picture that I made should demonstrate it:
You can learn all about articles and English (as well as how to do well in your IELTS test) here: https://ted-ielts.com/definite-and-indefinite-articles/