The word "foreskin" refers to the fold of skin on a man's private parts that is removed during circumcision. Here Moses is referring to a spiritual circumcision. This means the people must remove the sin from their lives. (See: [[rc://en/ta/man/jit/figs-metaphor]])
The phrase "stiffen your necks" is a metaphor that indicates that the people were stubborn, and God wanted them to stop being stubborn. Alternate translation: "stop being stubborn" (See: [[rc://en/ta/man/jit/figs-metaphor]])