KORAIL PASS is a rail pass for overseas visitors to Korea. Korean nationals are not eligible. The current KORAIL page says the pass can be used on KORAIL services but excludes SRT, metropolitan rail and temporary tourist trains, so do not assume every train shown in a journey planner is covered.
The current adult prices shown by KORAIL are KRW 131,000 for a 2-day select pass, KRW 186,000 for 3 consecutive days, KRW 234,000 for 4 consecutive days and KRW 275,000 for 5 consecutive days. Before buying, price the KTX or conventional-rail trips you actually expect to take; point-to-point tickets can be cheaper for a light itinerary.
KORAIL currently requires a valid passport and credit card to purchase the pass. Seat reservations can be made from one month before the train departure date, with up to two seat reservations per person per day. When traveling, carry the valid pass and passport; if you reserved a seat, carry the reservation information too. KORAIL states that a screenshot of the pass is not valid.
The pass does not guarantee a seat on a sold-out train. Reserve high-demand KTX legs as soon as your schedule is firm, especially around weekends and holidays. Recheck the official pass page before purchase because prices, train coverage and reservation rules can change.