This is in fact the climax of the play. If you think of Act I as "boy meets girl," and Act II as "boy gets girl," then Act III is "boy loses girl." Once Romeo kills Tybalt, Juliet's cousin, any chance Romeo and Juliet might have had to overcome their parents' objections to their marriage is gone. Furthermore, because the Prince has ordered the families to stop feuding on penalty of death, Romeo must now flee.
Like all of Shakespeare's plays, most of Romeo and Juliet is written in blank verse. Occasionally, a character speaks in prose, or ordinary, unmetered language. Most often, Shakespeare uses prose to show a character's lower social standing. In Romeo and Juliet, for example, it is mostly the Nurse and servants who speak in prose. However, in Act III Scene I, Mercutio speaks in prose when he challenges Tybalt and when he is stabbed: "Tybalt, you rat-catcher, will you walk?" "Ask for me tomorrow and you shall find me a grave man." In these instances, abandoning blank verse shows the character's strong emotion. It is easy to imagine that someone who is dying might leave off with the metered lines!
Iambic pentameter is a form of poetry in which each line has ten syllables. The syllables make a pattern of "feet," or pairs, with a particular rhythm. An iamb is a foot with two syllables, the second of which is stressed. It makes a rhythm like a heartbeat--ta-DUM, ta-DUM, ta-DUM, ta-DUM, ta-DUM. We see this rhythm in the sonnet that Romeo and Juliet recite to each other in Act I, Scene V when they first meet:
If I profane with my unworthiest hand...
See if you can find an example of iambic pentameter in Act III Scene I.