A traveler crossing the International Date Line eastbound subtracts one day, or 24 hours, so that the calendar date to the west of the line is repeated after the following midnight.
Crossing the International Date Line westbound results in 24 hours being added, advancing the calendar date by one day.
The International Date Line is necessary to have a fixed, albeit arbitrary, boundary on the globe where the calendar date advances in the westbound direction.
Please see your voyage itinerary to see if and how you are affected by the International Date Line.