Under the category of those things that most people simply accept even though they don't know why they're there is the mail stop. This is a four-digit code that shows up after a ZIP code in a US postal address. That four-digit code designates a specific part of a mail delivery area.
A city might have a few ZIP codes to divide it into mail delivery areas. Those mail delivery areas are usually further subdivided into smaller areas, each with a mail stop. For example, the ZIP code of 33004 might have four mail stops: 33004-0001, 33004-0002, 33004-0003, and 33004-0004.
A college or university might have its own ZIP code, especially if the institution is within a city's boundaries. In this case, each department might be given its own mail stop. If the college's ZIP code is 66124 and the mail stop of the chancellor's office is 0100, then the full ZIP code of the chancellor's office would be 66124-0100.
In the case of a college or university or a large business that has its own ZIP code, the mail stop is a physical place as well as a set of numbers. The mail that is to be delivered to the chancellor's office in the above example will be delivered to stop 0100. In some cases, this is just the office; in other cases, it is the central sorting facility for something like a large college department. In this case, the location is literally where the mail stops.
This literal designation of the mail stop refers to the inner workings of a post office as well. The ZIP code 33004-0002 is labeled 0002 within the 33004 section of the post office sorting facility, and all the mail designated to be delivered to 33004-0002 "stops" there. The postal service will certainly deliver mail that does not list the last four numbers. Like the ZIP code, however, they help mail delivery folks sort and deliver mail more quickly and efficiently. Failure to include the mail stop in a ZIP code could delay delivery of mail.