This is the 6327th most frequent Korean word.
쓸데없는
Unnecessary.
In this sentence, '쓸데없는' is used as an adjective to describe '말'(words) to express that the things being said are unnecessary or worthless.
그는 항상 쓸데없는 말을 해서 분위기를 망친다.
He always ruins the mood by saying unnecessary things.
Here, '쓸데없는' modifies '소리'(talk/sound) as an attributive adjective to indicate that the talk referred to is irrelevant or needless.
그러니까 쓸데없는 소리를 좀 멈춰줄래요?
Can you please stop saying these unnecessary things?
In this context, '쓸데없는' is qualifying '일'(matters/task), conveying the notion that the matters mentioned lack purpose or importance.