Shabbat is the Jewish sabbath, the seventh day of the week, which begins at Sundown on Friday and continues until Sundown on Saturday. It is based on two biblical references. First of all, from Genesis 2:3, which indicates God created the heavens and the earth in six days, but rested on the seventh:
And God blessed the seventh day, and sanctified it: because that in it he had rested from all his work which God created and made.
Also, In Exodus 20:9-11, part of the Ten Commandments:
Six days you shall labor and do all your work, but the seventh day is a sabbath to the LORD your God. On it you shall not do any work, neither you, nor your son or daughter, nor your male or female servant, nor your animals, nor any foreigner residing in your towns. For in six days the LORD made the heavens and the earth, the sea, and all that is in them, but he rested on the seventh day. Therefore the LORD blessed the Sabbath day and made it holy.
The link below indicates that the Sabbath or Shabbat has taken on additional meaning: it is a time of reflection, stepping back from the world and all its concerns, and considering the goodness and greatness of God. The Jewish people of course consider themselves God's chosen people because of the covenant with Abraham, hence the title.