What is a hash value?

A "hash value" is not particular to Oracle. Rather, it is a computer science concept. I will briefly explain this concept, but you can read more about hash values at Wikipedia.

Data is often stored in an array, which is a set of consecutive chunks of memory. Oracle's caches are often predefined...

arrays. The array is not big enough to hold everything. So Oracle takes that thing and applies some mathematical function to determine which chunk of memory, or the cell in the array, will hold the thing. This mathematical function is called a "hash function." This function returns an index to the array, which is commonly called the hash value.