This is a legal concept. You have to be loyal to your employer. You can’t intentionally undermine their business, charge them for hours you didn’t work, etc. It doesn’t mean that you have to go out of your way to advance their ends.
Maybe the word "loyal" is confusing here. I'm not aware of any legal requirement for loyalty. There's a legal prohibition against fraud and such, though.
It is literally called a duty of loyalty in many legal systems. Something you could have easily confirmed with a quick Google search in half the time it took to write your comment.
