Eunuchs are men who cannot physically have children, either because of a birth defect or because they were made unable through surgery (castration). Jesus also used the word in a figurative way for those who choose not to marry (see Luke 14:26; 18:29; 1 Corinthians 7:7–8).
for the sake of the kingdom of Heaven: Jesus was not criticizing marriage. He meant that unmarried people often have more freedom for ministry (see 1 Corinthians 7:25–35).