to cut off a males foreskin.
It is usually done in muslim and jewish countries. America is the only non jewish or muslim country where most males are circumcised. It is also the best treatment for phimosis and infections like balanitis.
It also prevents masturbation. This is entirely psychological due to being made to feel unhappy and inferior by anti circumcision activists and most of the other authors on this page, saying that circumcision is mutilation and decreases sexual pleasure. It makes the penis less sensitive which does not mean less pleasure. It increases staying power for better orgasms and is more enjoyable for both man and woman.
if all the anti circumcision activists didn't write all the crap about circumcision on the internet, circumcised men would not masturbate any less than uncircumcised men.