Ambient Light refers to light already present in the scene, it can may usually refer to the natural light of the outdoors but can also refer to overhead artificial lighting indoors, or any light source the photographer typically has little control of or had not added to the scene.

Exposing for ambient light is completely done in camera due to having no control over the ambient light. Though the light can be modified which might affect exposure, exposing for the scene is done using a light meter, typically built-in, and then changing settings in camera to suit.

North facing windows and walls are really useful photographically as throughout the day you’re mostly going to see an even, none direct light regardless of where the sun is in the sky.
This is due to the way the Sun rises and sets, from east to west and due to being in the northern hemisphere, the sun moves through the sky further to the south as opposed to directly overhead.

As the sun light hits the surround area, the light will be reflected onto the subject, either through a window or directly. Due to the sun not directly lighting the subject the whole area becomes the light source.
This produces a very large and even light source regardless of the time of day.

This portrait of Katie was taken using a North facing wall directly behind camera.

The windows along with the reflected light shining through are acting as a giant light source creating a soft flat lighting on Katie.
Even under her jaw you can see slight shadows but they are very faint due to being filled in from the light.

Her face is free of any shadows and she is evenly light across the entire frame. This lighting will have remained very much the same throughout the day as the position of the sun, until sunset, would have had very little affect, hence the benefit of a north facing window.

In contrast, on this south facing wall you can see very direction lighting coming from camera right, the shadows, though softend from a cloudy day are contrasty and are only being filled in from the wall reflecting some light back onto Katie.

The difference within the shot give of 2 very different looks but where taken just an hour apart simply from how the ambient light was being used. The light here will also change throughout the day, the direction of the sun would have lead to a front facing light as opposed to side lighting Katie.

Arguably I would say this image is more interesting due to the directional light.

However, it is of course possible to create directional light with North facing windows; In this photo of Nicola, we have light coming from a windows the right side of her face.

Here Nicola is broad lit with the narrow side of her face in shadow, though the shadow is still reasonably lit due to the room being well lit thanks to the large window. The directional light creates interest in the picture.

The problem I had with this image was composition. Distracting background elements make the image look cluttered but Nicola is well lit, the white walls are reflecting enough light to fill the narrow shaded side of her face enough to still retain detail will creating depth and shape.

Here, again we have very directional light coming from a north facing wall. This time however, the room it was shot is was very large providing little for light to reflect off to fill in shadows. This could be remedied in post production with sensors in the modern day being able to capture more dynamic range, however it would result in loss of quality and would most likely take more time.

The fix was to simply add a reflector parallel to the window and tilted slighted toward camera to ensure light was not spilled on the backdrop and to create a small rim/back light on the bottle. In this instance, I used a gold reflection as opposed to white/silver this was to create a warmer light, as though from a fire or incandescent light. Using white/silver would have reflected the 5000k daylight, which would've worked however I wanted to play with a mix of white balance within the image.