Why is the {0} and {1} needed to show output?

Question

namespace SoloLearn
{
class Program
{
static void Main(string[] args)
{
int x = 10;
double y = 20;
Console.WriteLine("x = {0}; y = {1}", x, y);
}
}
} // is the {0} and the {1} needed or what is it for in the Console.WriteLine("x = {0}; y = {1}", x, y); line of code?