이를 악물고 하면 안 되는 것이 없다.
=> Be tough as a nail and go for it, then you can do anything.
악물다 (clench) is an extreme version of 물다, "to bite".
So the literal meaning of 이를 악물다 is "to clench one's teeth" as you would to brace yourself for pain (이를 악물고 참았다 = I clenched my teeth to endure it). But it's also used figuratively to mean preparing to face an adversity by toughening yourself(살기 위해선 이를 악물어야 해 = We got to be tough to survive).
It's similar to the English expression "grit one's teeth" or "clench one's jaw", but it's probably stronger in meaning.