Romeo wants his tears to turn into fire if his eyes find someone other than Rosaline desirable at the party.
This shows us that Romeo's love for Rosaline is a blind and unhealthy love.
Romeo is obsessed with Rosaline and does not accept even seeing someone else, nor considering another woman more beautiful than her.
The phrase he uses is:
"When the devout religion of mine eye Maintains such falsehood, then turn tears to fires".
This shows us how exaggerated Romeo's love is, since Rosaline doesn't even reciprocate his love.