A Singh is the common surname/middle name of a male who follows the Sikh religion. The name Singh is derived from the sanskrit word Siṃha which means "Lion". It was later adopted into Sikhism in 1699 as per the instructions of Guru Gobind Singh Ji to achieve equality for all Sikhs.
Although commonly a Sikh name it is still used by other Indian communities in and around India.
Also I've embedded a video below to explain what it means to be a Sikh. Enjoy.