I need explanation of this code and what does it mean by & in C++?

enum xyz
{
  a, 
  b, 
  c
};

int main()
{
  int x = a, y = b, z = c;
  int &p = x, &q = y, &r = z;

  p = z;
  p = ++q;
  q = ++p;
  z = ++q + p++;

  cout<< p << " " <<; q << " " << z;

  return 0;
}