What is the intended effect of the iambic pentameter?

It usually only has an effect if it is read aloud. Iambic pentameter affects how the rhythm of the line goes when it is read.

Some poetry is written to be read, and if it is just observed on a page, it loses some of its power.

Well that doesnt really explain what Iambic Pentameter is .....

Iambs are pairs of syllables which can be short and long, or unstressed and stressed. When spoken aloud, an iamb follows a "ba-DUM" pattern, with the first syllable being short or unstressed and the second syllable being long or stressed. In iambic pentameter, there are five iambs in each line, creating a "ba-DUM ba-DUM ba-DUM ba-DUM ba-DUM" sound which is very regular and rhythmic. The verses may also be designed to rhyme with each other, using a variety of rhyming schemes ranging from creating rhyming couplets to complex interconnected rhymes which unfold over the course of the composition.

ambic pentameter is a form of rhythm which appears in poetry, songs, and some prose compositions. It is most closely associated with poetry, especially English poetry, which lends itself very well to this particular form of rhythm. One of the most notable writers who worked in iambic pentameter was William Shakespeare, who was fond of it for both his sonnets and his plays, in which characters classically spoke in verse. (although Shakespeare also wrote in Pro's)

Many forms of meter use iambs, because they are easy and natural to say. When poets compose new work, they think about the way that syllables will sound together, looking for Words which harmonize and create the iambic pattern, whether they are working in iambic pentameter or another form of meter. If the syllables clash with each other, they can make the piece feel stiff, stilted, or unsettling, which can be undesirable unless it is a deliberate effect which is designed to evoke specific emotions in the reader or listener.