속이 시원해요 · 속이 다 시원하네요 · 속 시원한 (+noun)
to feel relieved, to feel a weight off one's chest (lit. the inside feels cool)
Example
하고 싶던 말을 다 하니까 속이 시원해요.
Now that I've said everything I wanted to say, I feel so relieved.
Word parts
속 'inside, gut' is where frustration is felt; when it clears, the inside feels cool and fresh.
Good to know
Also for satisfying news or a well-deserved outcome: 속 시원한 결말 'a satisfying ending'. Opposite feeling: 속이 답답하다.