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 improves low-light sensitivity.
In a phone with a camera sensor resolution of 20 megapixels or less, pixel binning is often a special low-light mode of the camera, which may be triggered manually or automatically. The trade-off is resolution (detail). For example, a 16 megapixel sensor might produce a 4 megapixel image in pixel-binning mode.
Some high-resolution camera sensors are designed with pixel binning as the default, such as a 48-megapixel sensor that normally produces 12-megapixel images. In these phones, pixel binning is only disabled in a special mode that produces very high-resolution images.