There are generally two kinds of parameters to a function, those by VALUE and those by REFERENCE. A VALUE parameter is one where the variable's value (it's contents) are passed to the routine, whereas a REFERENCE parameter is one where the variable's location is passed to the routune.
Here's an example:
void Test( int byValue, int& byReference )
byValue++; // this changes the local version, but doesn't affect the caller
byReference++; // this changes the caller's variable too
printf( "inside TEST value=%d, ref=%d\n", byValue, byReference );
// this prints 2 for both since they were passed in as 1 in our call shown below...
int byValue = 1, byReference = 1;
Test( byValue, byReference );
printf( "after TEST value=%d, ref=%d\n", byValue, byReference );
// this prints 1 and 2, because Test() changed the byReference one but not the byValue one.