Here's the context of those lines:
Romeo:
"What lady is that which doth enrich the hand
Of yonder knight?
Servant:
I know not, sir.
Romeo:
O she doth teach the torches to burn bright!
Her beauty hangs upon the cheek of night
Like a rich jewel in an Ethiop's ear;
Beauty too rich for use, for earth too dear.
So shows a snowy dove trooping with crows,
As yonder lady o'er her fellows shows.
The measure done, I'll watch her place of stand,
And, touching hers, make blessèd my rude hand.
Did my heart love till now? Forswear it, sight!
For I ne'er saw true beauty till this night."
In these lines, Romeo is infatuated with the unknown woman (Juliet), who is attending the Capulet ball with her family. He praises her beauty, comparing her to a shining jewel and a snowy dove among crows. He is so smitten with her that he declares his previous love for Rosaline to be insignificant.
Here is the irony: Romeo has spent most of the play waxing poetic about Rosaline, declaring that he is hopelessly in love with her and would die if he could not have her. In just a few lines he makes a complete about face, completely contradicting his previous sentiments and declaring Juliet to be the most beautiful woman he has ever seen.
This immediate switch reveals the fickleness and shallowness of Romeo's affections. Despite proclaiming his intense feelings for Rosaline, he is easily captivated by Juliet's appearance, and his previous passion quickly fades.