Microlens Array
A layer of tiny lenses, one above each photosite, that gathers light which would otherwise fall on the gaps between them.
In depth
Photosites cannot occupy the whole sensor surface because wiring and circuitry take space, so without microlenses a significant fraction of the incoming light would be wasted. Toward the frame corners the microlenses are usually offset slightly inward, to accept light arriving at an angle from the exit pupil.
Examples in practice
Offset microlenses at the sensor edges are why modern designs vignette less than early digital bodies with the same lenses.