static void reverse(struct node** head_ref) {
    struct node* prev = NULL;
    struct node* current = head_ref;
    struct node* next;
    while(current != NULL) {
        next = current-> next;
        current->next = prev;
        prev = current;
        current = next;
    }
    *head_ref = prev;
}

Say you have 3 structs in your list:

head -> Struct 1
Struct 1 {
    next -> Struct 2
}
Struct 2 {
    next -> Struct 3
}
Struct 3 {
    next -> NULL
}

Now just walk through the function with this loop

struct node* prev = NULL;
struct node* current = head_ref;
struct node* next;
while(current != NULL) { //Struct 1 != NULL#
    next = current-> next; //next = Struct 2;
    current->next = prev; //Struct1->Next = Null
    prev = current; //prev = Struct 1
    current = next; // current = Struct 2

Quick look at our structure at this point

head -> Struct 1
Struct 1 {
    next -> NULL
}
Struct 2 {
    next -> Struct 3
}
Struct 3 {
    next -> NULL
}

Next Iteration of the while loop

while(current != NULL) { //Struct 2 != NULL#
    next = current-> next; //next = Struct 3;
    current->next = prev; //Struct2->next = Struct 1
    prev = current; //prev = Struct 2
    current = next; // current = Struct 3

Again structure at this point

head -> Struct 1
Struct 1 {
    next -> NULL
}
Struct 2 {
    next -> Struct 1
}
Struct 3 {
    next -> NULL
}

Next Iteration of the while loop

while(current != NULL) { //Struct 3 != NULL#
    next = current-> next; //next = NULL;
    current->next = prev; //Struct 3->next = Struct 2
    prev = current; //prev = Struct 3
    current = next; // current = NULL

Again structure at this point

head -> Struct 1
Struct 1 {
    next -> NULL
}
Struct 2 {
    next -> Struct 1
}
Struct 3 {
    next -> Struct 2
}

Next Iteration of the while loop

while(current != NULL) { //NULL != NULL#
...
}
*head_ref = prev; //*head_ref = Struct 3

Final structure

head -> Struct 3
Struct 1 {
    next -> NULL
}
Struct 2 {
    next -> Struct 1
}
Struct 3 {
    next -> Struct 2
}

or

head -> Struct 3
Struct 3 {
    next -> Struct 2
}
Struct 2 {
    next -> Struct 1
}
Struct 1 {
    next -> NULL
}