A eunuch is a castrated man. His genitals have been removed. This can be cutting off the testicles. The term "castration" is sometimes used to refer to emasculation where both the testicles and the penis are removed together.
Over the millennia since, eunuchs had a lot of different functions in different cultures such as domestics, treble singers, religious specialists, government officials, military commanders, and guardians of women or harem servants. When eunuchs ran a harem, a monarch could be sure he was the one who fathered his wives' children since eunuchs were always infertile.
In Byzantine art, angels and eunuchs were often shown in a similar light and manner.