What does ** do in C language?

In C arguments are passed by values. For example if you have an integer varaible in main

int main( void )
{
    int x = 10;
    //...

and the following function

void f( int x )
{
    x = 20;
    printf( "x = %d\n", x );
} 

then if you call the function in main like this

f( x );

then the parameter gets the value of variable x in main. However the parameter itself occupies a different extent in memory than the argument. So any changes of the parameter in the function do not influence to the original variable in main because these changes occur in different memory extent.

So how to change the varible in main in the function?

You need to pass a reference to the variable using pointers.

In this case the function declaration will look like

void f( int *px );

and the function definition will be

void f( int *px )
{
    *px = 20;
    printf( "*px = %d\n", *px );
} 

In this case it is the memory extent occupied by the original variable x is changed because within the function we get access to this extent using the pointer

    *px = 20;

Naturally the function must be called in main like

f( &x );

Take into account that the parameter itself that is the pointer px is as usual a local variable of the function. That is the function creates this variable and initializes it with the address of variable x.

Now let’s assume that in main you declared a pointer for example the following way

int main( void )
{
   int *px = malloc( sizeof( int ) );
   //..

And the function defined like

void f( int *px )
{
    px = malloc( sizeof( int ) );

    printf( "px = %p\n", px );
}

As parameter px is a local variable assigning to it any value does not influence to the original pointer. The function changes a different extent of memory than the extent occupied by the original pointer px in main.

How to change the original pointer in the function? Just pass it by reference!

For example

f( &px );
//...

void f( int **px )
{
    *px = malloc( sizeof( int ) );

    printf( "*px = %p\n", *px );
}

In this case the value stored in the original pointer will be changed within the function because the function using dereferencing access the same memory extent where the original pointer was defined.