Circumcision is the act of cutting off the foreskin of the male genitalia. In the Bible, this was an important act for Jews to enter into the covenant of God's people.
It can be said to be the biggest feature that distinguishes them from Gentiles, and the expression "uncircumcised" appears throughout the Bible as a derogatory term for Gentiles.
However, after Jesus came, it became clear that physical circumcision is not important, and only faith is the standard for being God's people.
Paul's arguments related to this are well presented in Romans 3 and Romans 4.