The International Date Line is located at about 180° longitude. When you cross the IDL from west to east, you subtract a day, and when you cross it from east to west, you add a day. It intentionally zigzags to avoid crossing through any countries.
Why the IDL zigzags
The IDL doesn't follow the 180° meridian exactly. It deviates eastward around Russia's Chukotka Peninsula so that all of Russia is on the same calendar day. It deviates westward around the US state of Alaska (specifically the Aleutian Islands) for the same reason. It also detours around Kiribati, which straddled the IDL until 1995 when the country shifted the IDL eastward so all of its islands would share the same date. This is why Kiribati's easternmost islands (the Line Islands) are in the UTC+14 time zone — the furthest ahead of UTC in the world.
What happens when you cross the IDL?
If you're flying from Tokyo to Los Angeles (crossing eastward), you "lose" a day — your departure on Monday afternoon arrives on Monday morning, and you've effectively gone back in time by a calendar day. If you're flying from LA to Tokyo (crossing westward), you "gain" a day — you depart Monday morning and arrive Tuesday afternoon. The actual elapsed flight time is the same; only the calendar date changes.
Countries near the IDL
Countries that sit near the IDL include Kiribati (UTC+14), Tonga (UTC+13), Samoa (UTC+13), and the US state of Hawaii (UTC-10). The Marshall Islands, Federated States of Micronesia, and Papua New Guinea are also nearby. The IDL ensures that each country's civil calendar aligns with its neighbors rather than being arbitrarily split.