If you have read any of Shakespeare's other sonnets—or indeed, any sonnet in the traditional form—you may recognize this meter. It is iambic pentameter, a meter in which each line contains five iambic feet, or iambs.
An iamb is an unstressed beat followed by a stressed beat (or emphasized syllable). So, iambic pentameter does not mean that there are only five syllables in a line, but that there are five points of emphasis. Let's look at a line from this poem poem:
That TIME of YEAR thou MAYST in ME beHOLD.
The capitalized syllables represent the places where we'd give stress to the sentence if we were reading it aloud.
Now, the meter in this poem is perfect—all the lines conform exactly to the iambic pentameter guideline. However, sometimes we will find a poem written in iambic pentameter where one or two of the lines doesn't quite fit—maybe it has four iambs, or six. In such a case, we would still describe the poem as being written largely in iambic pentameter.