of the heart; mental마음의 is the possessive form of 마음 (heart, mind, or feelings) and comes before a noun. Its exact translation depends on context, as in 마음의 준비 (mental preparation) or 마음의 약 (comfort for the heart).
KO: 여행을 떠나기 전에 마음의 준비를 하세요.
EN: Prepare yourself mentally before leaving on the trip.