A phrase commonly used in northern England, it derives from Old Yorkshire dialect.
It is a phrase used for greeting, although it is laregely unknown as to why the terms "now" (meaning this instant) and "then" (meaning in the past) were put together to form another way of simply saying "hello".
Usually pronounced "Nah-then".
Now then Jack, ya doylum. How ist? Whats tha bin doin with thisen?