If I zoom all the way out on Google Maps, just north-east of Australia is this goat-looking line that runs perpendicular across the equator. It's unlabeled.
It is the International Date Line (IDL). The "goat-looking line" is where it deviates around Kiribati.
The IDL is roughly based on the meridian of 180° longitude, roughly down the middle of the Pacific Ocean, and halfway around the world from the Greenwich meridian. In many places, the IDL follows the 180° meridian exactly. In other places, however, the IDL deviates east or west away from that meridian. These various deviations generally accommodate the political and/or economic affiliations of the affected areas.