The International Date Line is not, of course, a real thing. It is a conceptual necessity when accounting for multiple time zones around the world. It is an imaginary line, running down the Pacific Ocean from North to South, which determines where a calendar day ends and the next day begins.
This imaginary line runs roughly along the 180° meridian - with a few slight diversions here and there, intended to ensure that no country in this region is split into two. If you cross the International Date Line on a journey towards the east, then 24 hours are subtracted from your time. If you cross it when going west, then 24 hours are added to your time. Similarly, for every 15° crossed when heading east, you earn an hour and for every 15° crossed when heading west, you lose one.