This is the 39th most frequent Korean word.
이를
"이를" means "teeth" or "this" depending on context.
Here, '이를' refers to 'teeth' in the sense of clenching one's teeth.
그는 이를 악물고 힘든 일을 버텼다.
He gritted his teeth and endured the difficult work.
In this sentence, '이를' means 'this matter' or 'it' as the object of resolution.
이를 해결하기 위해 우리는 계획을 세웠다.
We made a plan to resolve it.
Here, '이를' again means 'this' or 'this objective' as the object of that effort.