Answer:
Star-crossed lovers indicate that they love each other at first sight. Death-mark'd love is fore shadowing that their actions later on affect the stories outcome. Because Juliet's parents wanted her to marry Paris instead of Romeo, she got a potion from Friar Lawrence which made her appear dead for 2 days. Romeo thought she had died and drank a poision to kill himself, and Juliet woke up to that. It is foreshadowing these events.