Lysander says, "The course of true love never did run smooth" to his love, Hermia, to comfort her since her father wants her to marry Demetrius (I.i.136). He's basically telling her that she should not worry about the situation because true love is supposed to face obstacles. What is being implied with this sentence is that if they didn't face trials for their love, then maybe it wouldn't be true love. (Lysander is certainly the hopeful romantic in the situation.) The sentence might also apply to Demetrius and Helena because they, too, go through trials before being able to embrace their love as well. The sentence is also a foreshadowing of those troubles to be acted out in the play as the lovers are manipulated by society and fairies alike; until finally, they are married to the correct person at the end of the play.