** stands for pointer to pointer as you know the name. I will explain each of your question:
what is this (**)?
Pointer to Pointer. Sometime people call double pointer. For example:
int a = 3;
int* b = &a; // b is pointer. stored address of a
int**b = &b; // c is pointer to pointer. stored address of b
int***d = &c; // d is pointer to pointer to pointer. stored address of d. You get it.
how is it represented in memory?
c in above example is just a normal variable and has same representation as other variables (pointer, int ...). Memory size of variable c same as b and it depends on platform. For example, 32-bit computer, each variable address includes 32bit so size will be 4 bytes (8x4=32 bit) On 64-bit computer, each variable address will be 64bit so size will be 8 bytes (8x8=64 bit).
what use does it have?
There are many usages for pointer to pointer, depends on your situation. For example, here is one example I learned in my algorithm class. You have a linked list. Now, you want to write a method to change that linked list, and your method may changed head of linked list. (Example: remove one element with value equals to 5, remove head element, swap, ...). So you have two cases:
1. If you just pass a pointer of head element. Maybe that head element will be removed, and this pointer doesn't valid anymore.
2. If you pass pointer of pointer of head element. In case your head element is removed, you meet no problem because the pointer of pointer still there. It just change values of another head node.
You can reference here for above example: pointer to pointer in linked list
Another usage is using in two-dimensional array. C is different from Java. Two dimensional array in C, in fact just a continuous memory block. Two dimensional array in Java is multi memory block (depend on your row of matrix)
Hope this help :)
