Calling a function within itself is simple. Think of the classic example: factorials.
n * (n-1) * (n-2) * (n-3)
until the terms reach 1. E.g.:
4! = 4 * 3 * 2 * 1
6! = 6 * 5 * 4 * 3 * 2 * 1
In C# code (I refuse to write Python, it's a toy language)
int Factorial(int n)
if (n <= 1) return 1;
return n * (Factorial(n - 1);
The function Factorial calls itself to generate the sequence. Each time it is called, it is passed a parameter which is one lower than it was lasts time, until it reaches the end, when it returns 1 instead.
Returning 1 is the "base case", the rest is the "recursive case".
Try it on paper, and see what happens. It's pretty obvious when you get your head around it.