Christianity started as a movement within Judaism, and later became independent. As a result, many scriptures and ideas are present both in Judaism and Christianity. Since about the 19th century, the term Judeo-Christian is used, to either describe this common heritage, or to show how Christianity developed from Judaism. The term Abrahamic religions also includes other religions, for which Abraham was an important person, most notably Islam.
At the time of Christ, Judaism was legal within the Roman Empire, and Jews were permitted to worship their religion. Christians were not covered by this arrangement, yet they had in the first century A.D. no Bible: the first book of the New Testament was not written until about 100 AD, though even that date this is not precisely known. It is deduced from its internal content. The other books were written even later. That is why in modern Bibles over half the content is from the Jewish tradition.
For this reason the phrase "Judeo-Christian" says just that Christianity rose from Judaism, and much of its Bible consists of incidents taken from Jewish mythology or history as represented in the Torah.