It’s 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, which 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 IDL, travellers 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 glove. Likewise a person travelling eastward would find that one few days had elapsed than he had recorded, as happened to Phileas Fogg in “Around the World in Eighty Days” by Jules Verne.
The IDL can be anywhere on the globe. But it is most convenient to be 180 degrees from the defining meridian that goes through Greenwich, England. It is also 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. Over the years, the position of the IDL has changed several times. The most recent change in the line was in 1995 when Kiribati moved a large segment of it to the east, so that the entire nation would be on the same side of the IDL. As with all other changes in the IDL, the change was made by a government with local interests. As a result, the line is as far as 150 degrees farther east then Honolulu. The position given on most maps is the line drawn by the British Admiralty in 1921.