"Star-cross'd" refers to the idea that Romeo and Juliet had so little chance of being together, it took the stars aligning (a rare occurrence) to get them together. They are from two households that are at war with each other, which is not exactly a recipe for romance. That is the "ancient grudge" mentioned in the prologue. Basically, they needed Fate to intervene in order to meet and fall in love.
"Death-mark'd love" is pretty self-explanatory; it means that their love is marked by death. What this comes down to is that their love is not meant to last. Fate will not allow it; it is not meant to be. This is settled at the end of the play. In the prologue, it mentions their deaths; in the lines, "a pair of star-cross'd lovers take their life," and "do with their death bury their parents' strife" foreshadow this occurrence.
I hope this helps!
