The & means that the function accepts the address (or reference) to a variable, instead of the value of the variable.
For example, note the difference between this:
void af(int& g)
{
g++;
cout<<g;
}
int main()
{
int g = 123;
cout << g;
af(g);
cout << g;
return 0;
}
And this (without the &):
void af(int g)
{
g++;
cout<<g;
}
int main()
{
int g = 123;
cout << g;
af(g);
cout << g;
return 0;
}