Google Pixel features an advanced autofocus system that utilizes two of the leading AF technologies: Phase detection autofocus, alongside laser detection autofocus.
We've first seen the Laser Autofocus system on the LG G3 device, and then on the LG G4, LG G5, Sony Xperia XZ, OnePlus 2 and others. The Laser Autofocus is good for photos where the subject is close to the camera, but much less effective when the subject is far away from the camera because of the receiving sensor size, which is very small. So it's less effective let's say, for landscape photography.
With this technology, the Google Pixel camera emits an infrared laser, which bounces on subjects in front of the camera and then received by the Laser AF system receiver on the phone. The AF system calculates the time it took the laser to travel until it reached the object and came back and because the laser speed is constant, the distance of the subject can be known. This focusing technology can focus on a subject in less than a third of a second, which is very fast and it's also proven to be useful when taking photos in low-light.
Here you can see the laser AF system in action on the LG G5:
The next AF technology is called phase-detection autofocus. In this technology, the sensor has what is called phase-detection pixels on the camera image sensor, each one is constructed of two sub-pixels, one for the color information and the other for the phase-detection AF. This also means that those specific PDAF pixels, receive half of the illumination. When the light strikes the phase-detection pixels, the camera can measure the phase differences and check if they are out of phase with one another. Phase-detection is proven to be very fast for subject-tracking and when continuous refocusing is required, but less effective in low-light conditions where the differences in contrast in the scene are low, and this makes it hard for the camera to do it's calculations in determining whether the compared images are in focus or out of focus.
According to the official Google Pixel page, Google Pixel, and Pixel XL cameras utilize both autofocus technologies as well as feature a camera image sensor with relatively large 1.55μm pixels, which should provide the camera with super fast and accurate AF system under various lighting conditions. All these technologies are for the rear-facing camera, the front camera features a fixed-focus, so all these advanced technologies work only with the rear-facing camera.