Think of a five-footed millipede strolling unsteadily along a winding path muttering dee-dum, dee-dum, dee-dum, dee-dum, dee-dum.
AND YOU'VE GOT IT! Apart from me sounding like a lunatic, the above image should help you to retain all you need to know about what is an unneccasarily complicated-sounding name for a line of FIVE BEATS.
Here's a quote from Sonnet 18, one of Billy Wobbledagger's finest (though not his best titled):
'SHALL I COMPARE THEE TO A SUMMER'S DAY?'
I dunno, shall you? Ah go on then...
There are FIVE BEATS in this line. Like so -
'Shall I comPARE thee TO a SUMmer's DAY'
Dee DUM dee DUM dee DUM dee DUM dee DUM
The 'dum' part is the thumped beat, the 'dee' part is the, what is called, 'unstressed' bit. It's the bit Shakespeare is less inclined to thump down on, WHACK. There are either beats (or 'stressed') words/syllables, or 'unstressed' words/syllables.
One 'dee DUM' is called a 'foot' (think of the millipede) and there are many types of 'feet'. Some are 'dee dee', some are 'dee dee dum', others are 'dum dum'. Each type of foot has it's name and this one (dee dum) is called, rather boringly, an IAMB. Shakespeare preferred this kind of foot. He loved an IAMBIC FOOT.
Furthermore, because there are FIVE IAMBS, it's called a 'PENTAMETER' which is, in other words 'METER OF FIVE'.
**TA DA! IAMBIC PENTAMETER.**
What's so special about it?
Well, the Elizabethans and Jacobeans loved to write in iambic pentameter because it is the closest metre to the natural rhythm of human speech.
Why did Shakespeare use it?
Well, he used it to demonstrate intellectual flair, to order his ideas, and to create beautiful rhythms and sound patterns.
How can it help me?
By identifying where something is stressed or unstressed you can get hints and clues about what Shakespeare was intending and how he wished for his stories to be told, or his sonnets interpreted. You can use your SWANKY TECHNICAL KNOWLEDGE to prove awesome points. For example:
The iambic pentameter does not stress 'shall' in the opening line of Sonnet 18. This is because Shakespeare isn't asking his subject for permission to 'compare' him to a 'Summer's day', he is going to do it anyway, as an offering. Like Summer, the moment will not wait, it must proceed regardless and if it isn't seized, it could wither and die. Using iambic pentameter in this way, Shakespeare creates a sense of spontaneity.
Hope this helps! Good luck!