The International Date Line is an imaginary line set up by the people, and it is needed so that the people manage to organize the time more properly on the whole planet.
This line is halfway through on the other side of the planet from the Prime Meridian, also known as the Greenwich Meridian, so since the Prime Meridian is at 0 longitude, the International Date Line is roughly at 180 degrees of longitude.
As I said, roughly 180 degrees of longitude, so this line is not a straight one, but instead it moves a bit in both eastern and western direction at certain places. The reason why it is not straight is because it follows the borders of the countries located in the Pacific through which it passes or touches their borders, in order to not divide their time.
