This is the 345th most frequent Japanese word.
国内
Domestic; within the country.
国内 here means 'within the country', referring to Japan as the domestic region being discussed.
日本国内では観光地が多いです。
Within Japan, there are many tourist spots.
国内 in this context modifies 'line' to specify 'domestic flight' as opposed to an international one.
国内線の航空券を予約しました。
I booked a domestic flight ticket.
国内 contrasts with 'abroad' (国外), indicating the locality activities within the country.