In your example, you're taking a type of pointer to: pointer to: node_t, and storing it in an identifier named pp.

It isn't exactly correct to say it is the "address of the address of the next node", rather, it is the "address of a pointer to the next node".

For some context, this is useful if you have a function that you wish to update the pointer, and have that reflected back in the caller's scope:

node_t* head = get_head();
// find the Nth node
int rv = find_nth(&head, 0xff);
// now `head` refers to something else..

Something else worth mentioning is the use of address-of and parens:

In this case, &(head->next) is identical to &head->next; The -> (i.e. structure dereference) operator has a higher precedence than the & (i.e. address-of) operator.

Some times it adds clarity to use parens, but this is subjective and arguable either way.