Digital camera sensors consist of a number of pixels that collect photons (energy packets of light), which are converted into an electrical charge by the photodiode.
In turn, this is converted into a digital value by the analog to digital converter (AD converter). This finally allows the camera to process these values into the final image.
The fill factor relates to the size of the photodiode in relation to the surface of the pixel. Many digital cameras have a small or low fill factor, as modern DSLRs have more per pixel circuitry.
To compensate for this, an array of "microlenses" is placed over the color filter array to funnel the photons of a larger area into the smaller area covered by the photodiode.