Iambic pentameter is a meter with five unstressed and stressed syllables in a row. Since it says pentameter, it means there are 5 meters, and since 1 meter consists of 2 syllables, that means that one line consists of 10 syllables, which go: unstressed, stressed, unstressed, stressed, etc. until you reach the 10th one.
For example:
When I do count the clock that tells the time,
To swell the gourd, and plump the hazel shells.
