From Wikipedia:

"Various groups have considered themselves chosen by God for some purpose such as to act as God's agent on earth. This status may be viewed as a self-imposed higher standard to fulfill God's expectation.

Specifically, in the Hebrew Bible, called the Old Testament by Christians, and the Tanach by Jews, the phrase Chosen People refers to the ancient Hebrews/Israelites. In the Book of Deuteronomy, God proclaims the Nation of Israel, known originally simply as the Hebrews, as His chosen people above all others.(Deuteronomy 7:6) As mentioned in the book of Exodus, the Hebrew people are God's chosen people and from them shall come the Messiah, or redeemer of the human race. The Israelites also possess the "Word of God" and/or "Law of God" in the form of the Torah as communicated by God to Moses. Jews and, by extension, Christians consider themselves to be the "chosen people" Adherents to Islam make, by the same extension as Christians, the same claim of chosenness by accepting what they see as the validity of the Law of God as told by Moses; as do other religions that are built on those same laws."