재수가 · 재수를
studying a year to retake the college entrance exam
Example
원하는 대학에 떨어져서 재수를 하기로 했어요.
I didn't get into the college I wanted, so I decided to retake the exam next year.
Word parts
재 again+수 study
Literally 'studying again'.
Good to know
Don't confuse it with 재수 'luck': 재수 없다 means 'unlucky' or, rudely, 'obnoxious'.