Basically it means if you ever had two functions:

function twotimestwo() {
   return 2*2;
}

function twotimesfour() {
   return 2*4;
}

you should try to refactor them such that:

function twotimestwo() {
	return twotimes(2);
}
function twotimesfour() {
	return twotimes(4);
}
function twotimes($num) {
	return 2*$num;
}