iambic pentameter -
iambic because each bit (it's called a 'foot') is made up of two syllables: dee-dah (called an 'iamb')
pentameter because there are five of these feet in each line
For example, from Portia's speech in the Merchant of Venice,
Different editions have 'heaven' or "heav'n", but it's clear that Shakespeare wanted the actor to pronounce 'heaven' as one syllable.The qua/lity / of mer/cy is / not strained
It dro/ppeth as/ the gen/tle rain /from heav'n'
In this case, there's a convention in all poetry that 'heaven' is usually pronounced as one sylable (find a site with hymn tunes on it); but in other cases Shakespeare would often drop a syllable so that the elided word would fit in to the metre.
(Note for the interested: by no means all his plays are in iambic pentameter. Sometimes he changes from prose to verse as a signal of what sort of character it is: a king would speak in verse, but a porter wouldn't. Sometimes a person starts to speak in prose, but it becomes verse-like (it has the rythm, but isn't laid out as verse) to mark a change of mood.)