What are Bayer color filters?

Bayer color pattern defines the order in which the three primary colors (red, green, and blue) are arranged on a digital sensor color filter.
Usually, single-sensor color cameras use a monochrome sensor with a color filter applied on top: Bayer pattern defines which color cover each sensor's pixel.
Once the is acquired, a subsequent demosaicing process reconstructs the entire image by interpolation. This way it's possible to get color images with a single image sensor.