This is the 2nd most frequent Korean word.
수
The Korean word "수" can mean "number," "way," "possibility," or "ability," depending on context.
Here, "수" is used in the idiomatic phrase "어려운 수가 있다," meaning a situation or case that is difficult.
수학 문제를 푸는 것은 어려운 수가 있어요.
Solving math problems can sometimes be difficult.
In this sentence, "수" is used as part of the grammatical construction "-을 수 있다" which indicates ability or possibility.
그는 노래를 잘 부를 수 있어요.
He can sing well.
"수" appears in the idiomatic expression "어쩔 수 없다," meaning there is no way to avoid or change something.