Xiaomi has decided to go with a dual-camera setup in the Mi 5X. This means that there are two cameras as the back, each one with its own sensor and lens. The first lens is a 12-megapixel wide-angle f/2.2 lens with 1.25-micron size sensor pixels and the second camera features a 12-megapixel sensor with a f/2.6 aperture lens and a 1-micron pixel sensor. This is a similar setup like the Mi 6 dual-camera setup but slightly different.
One of the benefits of this dual-camera setup is that users can choose which camera to use. For example, they can choose to shoot with the wide-angle camera for landscape or indoor pictures or change to the telephoto lens for portraits. Due to this configuration, the Xiaomo Mi 5X rear camera also features a "Portrait" mode, which gives you the option to shoot portraits with a shallow depth of field effect (also known as "Bokeh"). In this mode, the subject appears sharp, while the background appear blurry, which helps to bring the subject in the image out and blur out distracting background elements.
One of the downsides of this camera is that the telephoto lens is slower than the wide-angle camera, having a slower aperture lens and smaller pixels. Furthermore, the 4-axis OIS works only for the wide-angle camera for what I've heard. Generally speaking, it's better to use the wide-angle lens when shooting in dark situations for better exposure and better image quality. The reason for that is that when shooting in low light, the telephoto camera will force to use a higher ISO sensitivity to lack of light to produce a well-exposed image. I did read that the telephoto camera comes with "dark light enhancement technology" and "high dynamic range adjustment technology". I don't how much improvement you can get using those technologies. We need to see some real life test to see how they work in practice to improve the image quality in low-light performance.
Now for a bit more detailed information about the Xiaomi Mi 5X dual-camera. The wide-angle lens has 5 elements and an equivalent (in 35mm terms) focal length of 26mm. The Telephoto camera has an equivalent focal length of 50mm and it's also made of 5 elements. so this is close to 2x zoom (1.92x to be exact). Of course the camera doesn't zoom through the focal lengths, you can either choose to use the first camera or the second one interchangeably.