By describing Romeo and Juliet as “star-crossed lovers,” Shakespeare establishes the fact that they have to be together, and they were fated to fall in love.

Although there is a struggle between choice and fate going on in this play, the prologue establishes an interesting idea.  In short, it seems to be saying that in order to end this feud between the two families, their children had to die.  In order for them to die, they had to fall in love.

A pair of star-cross'd lovers take their life;

Whose misadventur'd piteous overthrows

Doth, with their death, bury their parents’ strife.

The fate of Romeo and Juliet is in the stars.  They were destined to fall in love.  They were destined to meet tragedy.  They had no control over the actual events themselves.  That is what “star-crossed” means, and Shakespeare wanted us to understand this from the very beginning.

It definitely does seem as if fate is against the young lovers.  First, they are born into feuding families; then, for Romeo accidentally gets invited to a party and sees Juliet.  Then, Romeo accidentally kills Tybalt in a duel and the two lovers are separated when Romeo is banished.  Finally, Romeo does not get the note about Juliet’s death being faked, and kills himself just before she wakes up.  Ouch, that’s a run of bad luck.

A pair of star=crossed lovers take their life,
Whose misadventured piteous overthrows
Do with their death bury their parents" strife. (1.1.6-8)

In the Prologue of Romeo and Juliet that is spoken by the Chorus, who introduce the play to the Elizabethan audience, the term "star-crossed" would be one quite familiar to these Elizabethans. For, the stars are part of the Chain of Being. When one part of this chain is upset, as in the stars, then there is disorder and chaos. So, when the stars are taken out of their order, things go awry and fate changes the order of things.

Therefore, when Romeo and Juliet have their stars taken out of order and "crossed," they become fated lovers, lovers to whom misfortune will come. Thus, their lives contain a destiny that will prove tragic for them. In modern times, one would say that they have bad luck, such as when they meet and their families are in a feud against one another; or, when Romeo happens upon Mercutio and Tybalt fighting and he inadvertently kills Tybalt; or, when John cannot get the Friar's message to Romeo because Mantua is quarantined; or, when Friar Laurence runs out of the catacombs and Juliet is left alone to her fate.

The story of Romeo and Juliet, traditionally listed amongst the Tragedies of Shakespeare, more specifically can be considered a "disaster!"  The phrase "star -cross'd lovers" of course refers to them both originating from warring households, that no matter what they felt for each other, the hard politics between their families would impede their love for each other, and cause the disaster of their suicides.  "Disaster" is a Greek word, "Dis" meaning "against" and "Aster" meaning "star...."

To a Shakespearian audience, more versed in astrology and the effects the heavens had upon humans and their affairs than we are, the phrase "star-cross'd" would suggest to them a bit of foreshadowing of the trouble ahead.

This phrase refers to the idea that the stars determine people's destinies, like in astrology.  So saying that they are "star cross'd" is saying that they have been given an unlucky fate or destiny.

From there, you should be able to see why they are star cross'd.  They are unlucky in who they fall in love with.  They are unlucky in Romeo being unable to leave the city like he is supposed to.  They are unlucky in having him get to the tomb before Juliet wakes up.

So Romeo and Juliet are unlucky all the way around.  That's why Shakespeare uses this figure of speech to describe them.

The term "star-crossed" refers to the idea that those involved will be impacted in a negative way. The term generally refers to a couple that due to some unfortunate circumstance, some uncontrollable, pre-determined fate are destined for failure.

In Romeo and Juliet, the title characters are impacted by many factors that are beyond their control, primarily caused by family feuding. The "bad-luck" that plagues the relationship from the beginning foretells a sad tale of youth, conflict and an attempt to be together despite the painful realities of their situation. In the end, the couple falls to an eternal rest; while those who love them mourn their death.

Romeo and Juliet is not the earliest example of star-crossed lovers, but it is among the most famous.

The term means that they are doomed by fate.  The whole idea of astrology (which was much more believed in back then) is that the stars have something to do with what happens to us.  Therefore, if someone is "star crossed" they have been dealt a bad hand by fate.

This has happened to Romeo and Juliet because they have had the bad luck to be born to families that hate one another.  This means that, when they fall in love, they cannot just get married like other people might.

There are numerous other instances of bad luck in the rest of the play, but this is the main one.

So -- bad luck = star crossed.