To circumcise is to remove the prepuce or foreskin off of a penis. In the Old Testament times, it was used as a holy ritual to consecrate or set apart those who followed God. It was established as a command for the nation of Israel. (Some cultures practice female circumcision, though this isn’t commanded in the Bible).
The practice did not continue after the death and resurrection of Jesus, as circumcision was no longer deemed necessary. (Even so, some Jewish converts to Christianity, wrongly tried to impose it on others.)