It means 'Calm down', as Izzie explained. But said to anyone but a child, it is almost always a form of mild criticism as you never normally address someone as 'Tiger'. Saying it to an adult, is like saying 'Hey, calm down, you're letting your emotions get to you (like a child)'.

It's an American colloquialism. It means calm down. You'd say it to someone who is overly excited.