A type of camera lens that can adjust the distance of what is in focus (focal length).
Auto-focus usually provide better photo quality than fixed-focus, since auto-focus lenses can be larger and allow more light to reach the camera sensor. The downside of auto-focus is usually a delay when taking a photo (shutter lag).
Unlike fixed-focus lenses, auto-focus lenses provide "depth of field", meaning objects not at the focus distance appear out of focus (blurry).
Most auto-focus cameras mechanically change the distance between two lenses to do this using a small motor or ultrasonic actuator. Newer technologies use lenses that can change shape, providing the same functionality in a smaller package.
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 edges appear sharp.
More advanced auto-focus systems in phones use lasers and/or dedicated focus pixels on the main sensor. A system using focus pixels is known as phase detection (PDAF).