Why is the verse an example of iambic pentameter

You haven't provided the verse you were given, but I can answer your question anyway.

A pentameter is a type of a verse line that consists of five metrical feet.

One type of metrical feet is called an iamb. One iamb consists of an unaccented (unstressed) syllable followed by an accented (stressed) syllable.

Based on this information, we can conclude that the iambic pentameter is a line that consists of five iambs. It was often used by William Shakespeare. The following verse from Shakespeare's A Midnight Night's Dream is a good example of iambic pentameter:

And I do love thee: therefore, go with me;

I'll give thee fairies to attend on thee,

And they shall fetch thee jewels from the deep,

And sing while thou on pressed flowers dost sleep

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