An imaginary line through the Pacific Ocean roughly corresponding to 180° longitude, to the east of which, by international agreement, the calendar date is one day earlier than to the west.
The International Date Line is the imaginary line on the Earth that separates two consecutive calendar days. That is the date in the Eastern hemisphere, to the left of the line, is always one day ahead of the date in the Western hemisphere. It has been recognized as a matter of convenience and has no force in international law.
Without the International Date Line travelers going westward would discover that when they returned home, one day more than they thought had passed, even though they had kept careful tally of the days. This first happened to Magellan's crew after the first circumnavigation of the globe. Likewise, a person traveling eastward would find that one fewer days had elapsed than he had recorded, as happened to Phileas Fogg in "Around the World in Eighty Days" by Jules Verne.
The International Date Line can be anywhere on the globe. But it is most convenient to be 180° away from the defining meridian that goes through Greenwich, England. It also is fortunate that this area is covered, mainly, by empty ocean. However, there have always been zigs and zags in it to allow for local circumstances.