A hash is a short fingerprint calculated from information using a special function. A program can use it to compare files or help identify a particular version.
Imagine a machine that gives each document a compact mark based on its contents. If the contents change, the mark will usually change too, making it easier to notice a difference.
Hashes are not guaranteed to be unique: two different pieces of information can sometimes produce the same result. A hash is useful for checking and organizing, but it is not always a perfect identity label.