A hash value is essentially the checksum of something else (in this case a name). If the name is the same then the hash value will also always return the same value. On the other hand, a hash value is computed in a way to be as unique as possible.
Taking the name of an item, a surface or the combination of an item and surface name, this node will either create a colour or output a hash value.
The hash is identical to the one used by Cryptomatte.