Simplify the following text for a Grade 9 native English-speaking person.
Original: Incheon International Airport ( IIA ) is the largest airport in South Korea , the primary airport serving the Seoul national capital area , and one of the largest and busiest in the world .
Simplified: Incheon International Airport is the largest international airport in South Korea where all international flights depart and arrive everyday . Gimpo Airport located near city Seoul used to be the international airport of Korea .