A type of camera system that can automatically adjust its lenses to focus on subjects at different distances from the camera.
Auto-focus generally provides better photo quality than fixed-focus, since auto-focus lenses can have a larger aperture and therefore allow more light to reach the camera sensor. Unlike fixed-focus lenses, auto-focus lenses can provide "depth of field", meaning objects not at the focus distance may appear out of focus (blurry).
The most basic auto-focus systems use contrast detection (CDAF), which does an electronic version of what how you would focus with your own eyes: adjusting the lens until the edges of objects appear "sharp".
More advanced auto-focus systems in phones use lasers to measure the distance from the camera to the subject, and/or dedicated focus pixels on the main sensor. A system using focus pixels is known as phase detection (PDAF).
Most auto-focus cameras mechanically change the distance between two lenses to do this using a small motor or ultrasonic actuator.