Iambic pentameter does not symbolize anything. It is a rhythm. It was introduced by playwrights like Christopher Marlowe and the University Wits.
The rhythm is quite simple---de DUM, de DUM, de DUM, de DUM, de DUM. It alternates the stress. It was used because it most closely reflects the number of syllables we get said in a single breath. In other words, it most closely mirrors everyday English speech rhythms.
Ask any actor and they will tell you that verse is easier to learn than prose. This was important during the Elizabethan period because actors had to keep approximately 30-40 plays in their head. If someone requested to see a play they had not performed for a while, there was no brush up rehearsal.
Shakespeare used iambic pentameter as a norm upon which he could change the rhythm much like a jazz muscian.
It is true that for the most part, high born or noble characters use verse, and servants and lower class characters use prose but this is not always true. For example, in Romeo and Juliet, Mercutio, a kinsman of the Prince, speaks more prose than verse. In his case, he is speaking informally, he is chatting with friends.
A character like Caliban in The Tempest speaks verse although he would be considered a low born character. In his case, he learned to speak from Prospero.
There are no strict rules as for who speaks what. Iambic pentameter is a rhythm, nothing more complicated than that.