This is where there are lots of small begnign lesions which are usually black or brown in colour. They are most commonly found on the face and in people with darker skin colour.
They normally appear after puberty and can continue to grow in number.
No treatment is required, but some people like to remove them for cosmetic reasons. If needed they can be treated with cryotherapy and curettage.
References
https://en.wikipedia.org/wiki/Dermatosis_papulosa_nigra