A lenslet is literally a small lens. The fact that distinguishes it from a small lens is that it is part of a lenslet array. A lenslet array consists of a set of lenslets in the same plane. Each lenslet normally has the same focal length.
It has many uses but is commonly found in Shack-Hartmann wavefront sensors and beam homogenization optics for projection systems.
References
Lenslet Wikipedia(Text) CC BY-SA