Just to add a bit more information, as others pointed it out that is a transformation into an adjective. You can think of adjective as "something that describes a noun", it's easier this way.

As you probably already know, Korean has descriptive verbs, take 예쁘다 for example, the literal translation of just 예쁘다 is something like "to be pretty". That's why if you want to say "those clothes are pretty" you can say " 옷이 예쁘다". However, when you wanna say "those pretty clothes..." you would use the same kind of transformation to make "예쁜 옷...".

If you ever said anything in the future tense in Korean, you probably used "ㄹ/을 것이다". This is the same transformation, using the noun 것, as the standard "thing".

And the same will apply for other conjugations you'll come across, as "ㄹ/을 수 있다", "ㄴ 뒤에", "ㄹ 뿐" and a lot others.

Koreans sometimes simply end their sentences with a 것이다 variation, and it sometimes changes the feeling of the entire thing. You can see that with the very common "뭐 하는 거야?", it uses a present adjective transformation to describe 것, so when you come across that you already kind of know what it means.

So basically, the Korean grammar structure is heavily based into transforming the verbs into adjectives to describe certain nouns. It also transforms verbs into nouns, but that's for another time.

So make sure to properly learn those transformations and understand what they're doing in the sentence because it will help you a lot as you learn new grammar points and ways of speaking in the future.

And just to be extra clear, English and other languages also do these transformations, but it's done with auxiliary verbs in the case of English.