Eunuchs are men that can't have sex/reproduce......usually because they have been castrated. They were used a lot by Kings/Emperors in the past to guard Harems full of women because no one had to worry about them having sex with the women they were guarding.
That's why it's said in Isaiah....."Let not the Eunuch say, Behold, I am a dry tree".......and it then goes on to say something to the effect of "I will give them something better than the names of sons and daughters, even an everlasting name that shall not be cut off".