ויוצא אתו החוצה...ויאמר לו כה יהיה זרעך, "And (the Almighty) too "Avraham" outside... and he said to him, so, too, will be your descendants." (בראשית ט"ו:ה')
Rashi cites a statement from the Talmud which says that Hashem told אברהם to not believe the effects of astrological influence. Even if there is a sign in the stars that you are not supposed to have children, you will overcome this and have children. From here, the Talmud states, "There is no mazel for Israelites." This is the meaning of the second half of the verse, "So too will be your descendants." The Jewish people don't have to worry about negative predictions from the stars.
Some people are nervous if somebody predicts a negative future for them through astrology, cards, etc. If one trusts in Hashem and is aware of His power, then that will free a person from any fear of these predictions. Good deeds and prayers will be able to change a negative destiny into a positive one.