A feature of some camera sensors that combines multiple neighboring camera sensor pixels into one virtual pixel. Commonly, groups of four pixels (2x2) are combined into one.
This technique reduces noise and can improve low-light sensitivity.
The trade-off is resolution (detail). Using common 4-pixel binning, a 16 megapixel sensor will produce a 4 megapixel image.
In a phone, pixel binning may be employed by a special low-light mode of the camera, which may be triggered manually or automatically.