When you declare a variable as here
int a[10][10];
you are telling the C++ compiler that you want 100 consecutive integers allocated in the program's memory at runtime. The compiler will then provide for your program to have that much memory available and all is well with the world.
If however you tell the compiler
int x = 9001;
int y = 5;
int a[x][y];
the compiler has no way of knowing how much memory you are actually going to need at run time without doing a lot of very complex analysis to track down every last place where the values of x and y changed [if any]. Rather than support such variable size arrays, C++ and C strongly suggest if not outright demand that you use malloc() to manually allocate the space you want.
TL;DR
int x = 5;
int y = 5;
int **a = malloc(x*sizeof(int*));
for(int i = 0; i < y; i++) {
a[i] = malloc(sizeof(int*)*y);
}
a is now a 2D array of size 5x5 and will behave the same as int a[5][5]. Because you have manually allocated memory, C++ and C demand that you delete it by hand too...
for(int i = 0; i < x; i++) {
free(a[i]); // delete the 2nd dimension array
}
free(a); // delete a itself