Fast power function

#1

what is meant by fast power function?

#2 
ll power(ll x,ll y){
	if(y==1||x==0||x==1)return x;
	if(y==0)return 1;

	ll ans=power(x,y/2);
	if(y%2)ans=ans*ans*x;
	else ans=ans*ans;
	return ans;
}
``