To shake the dust off their feet when they leave. This instruction is found in Chapter 10 of the Book of Matthew. The Twelve in this question is referring to the 12 disciples. Those 12 disciples were: Thomas, Peter, James, Simon the Zealot, John, Philip, Andrew, Matthew (also known as Levi), Bartholomew (also known as Nathanael), Jude (also known as Thaddeus), James, the Lesser (also known as Younger), and Judas.
There were many instructions Jesus gave these 12, which includes not going around the Gentiles or entering a town of Samaritans. Along with that, Jesus instructed the 12 to heal the sick and drive out demons.