The International Date Line passes through the Arctic and Pacific Oceans.
The IDL is a byproduct of the decision to have a standardized "global" series of time zones, as opposed to earlier, more localized systems. Because a single day is, by definition, one rotation of the Earth, different places on the Earth will reach their completed day (midnight) at different times. This also means that, at some arbitrary point, locations to the east will be starting their day, and locations to the west will be finishing theirs. This location in the International Date Line.
The Date Line is not entirely specific; there is no governing body that oversees and arbitrates its boundaries, and since it is located in one of the less-populated regions of the globe, it doesn't cause significant economic or territorial disputes.