Most of Shakespeare's plays are written in a verse (poetry) style called iambic pentameter. Sounds kind of scary, so let's break it down.
An "iamb" is an unaccented syllable followed by an accented one. "Penta" means "five," and "meter" refers to a regular rhythmic pattern. So "iambic pentameter" is a kind of rhythmic pattern that consist of five iambs per line. It's the most common rhythm in English poetry and sounds like five heartbeats:
ba-DUM, ba-DUM, ba-DUM, ba-DUM, ba-DUM.
Let's try it out on this line from Troilus and Cressida:
"her BED is INDia, THERE she LIES, a PEARL."
FYI: The word "India" is pronounced here with only two syllables ("In-dya"), instead of three ("In-di-a").
So, who runs around talking like this? Princes like Troilus and other "upper-class" characters, that's who. The idea is that speaking verse fits their social rank.
But commoners and slaves (like Thersites) tend to just speak regular old prose. Here's an example:
The plague of Greece upon thee, thou mongrel
Nope. We don't catch princes like Troilus running around talking like that. This speech is reserved for the crude slave that has something nasty to say about everybody.