Romeo meets Juliet for the first time at a huge party thrown by the Capulets. He's instantly smitten by this beautiful young lady the moment he first lays eyes on her. He thought that Rosaline was the girl for him, but he now realizes that that was just a boyish infatuation; Juliet is the real thing. In fact, Romeo had seriously considered not going to the party because he was still pining over Rosaline and didn't want to bump into her. But that's all ancient history now. Rosaline is the past, and Juliet is the present and future.
When he sees Juliet for the first time, Romeo suddenly realizes that he'd never really known what beauty was until this moment:
Did my heart love till now? Forswear it, sight!
For I ne’er saw true beauty till this night. (act 1, scene 5)
Nor love, for that matter. Romeo genuinely believed he was in love with Rosaline, but he now knows that that wasn't really the case at all—the feelings he has for Juliet are completely different from anything he's ever experienced before. As he puts it himself, his heart had never loved until now.
Romeo goes on to describe Juliet's extraordinary beauty, likening it to a light that shines more brightly than a torch. He also compares it to a rich jewel shining in an Ethiopian's ear. There's clearly something very powerful and exotic about the emotions that Juliet's beauty has kindled in Romeo's lovestruck soul.