When, in "Romeo and Juliet", the title characters are referred to as "star-cross'd lovers," Shakespeare is foreshadowing or warning of their ultimately tragic destiny. Although Shakespeare coined the phrase, "star-cross'd," meaning ill-fated, for this play, it would have been understood at the time as a reference to astrology, or to the figuring of fate in the stars.
The phrase is used in the play's opening prologue, which is written as a sonnet — a common literary expression of love, especially conflicted love, during Shakespeare's era. Although the love affair of Romeo and Juliet has an unequivocally tragic ending, there is also the sense of it being fated in the stars, as it finally ends the bitter feud between their two families.
The full line from which the phrase is taken goes: "From forth the fatal loins of these two foes / A pair of star-cross'd lovers take their life."
Unusually for a prologue, the function here is not simply to introduce the audience to the play's setting, but to actually tell the audience what is going to happen. As a result, the play is given a fated, destined feel from the outset, from which the audience knows the characters cannot escape.