The hash code of x is an integer produced in a deterministic way from x, and perhaps from the hash codes of the contents of x. See hashing for a discussion of the requirements that the hash codes used here are designed to satisfy.
Hash codes may change from one version of Macaulay2 to the next, but changes to the hash codes of the basic types will be avoided, if possible. That includes lists, sequences, strings, hash tables, options, Boolean values, and numbers.
The object hash is a compiled function.