Apple's Portrait Mode has become a popular way of taking impressive shots using a depth-of-field effect known as bokeh, allowing iPhone users to shoot a photo that keeps the subject sharp with a blurred background.
To use Portrait Mode, open the Camera app and swipe to Portrait mode. Portrait Lighting effects will appear at the bottom of the viewfinder.
To change the focal length, tap the circular 1x button in the bottom-left of the viewfinder. 1x corresponds to the wide lens, and 2x switches to the telephoto lens.
You can see the difference between the two modes in the second and third images above, but in general the 2x mode seems to be better for capturing people, while the 1x lens is better for shooting smaller objects.
Apple has widened the aperture of the iPhone 11 Pro's telephoto lens to f/2.0 from f/2.4 in the iPhone X and XS. This allows more light to hit the sensor, which should translate to better Portrait Mode results in lower lighting conditions.
Don't forget, you can now also switch to the front-facing camera and get a selfie in Portrait Mode, too.