Please please define 'extra syllable'.This phrase was used in some book about the stylistic analysis of Shakespeare's plays. Is it synonymous with 'feminine ending'?
Feminine rhyme is when a word at the end of a line of poetry rhymes in two or more of the syllables:
A woman’s face with nature’s own hand painted,
Hast thou, the master mistress of my passion;
A woman’s gentle heart, but not acquainted
With shifting change, as is false women’s fashion...
Notice how the word "painted" rhymes with "acquainted".
When you have a feminine ending at the end of a line of iambic pentameter, as in the example above, there is an extra syllable. If the lines above were true to iambic pentameter, there would only be 10 syllables to the line, but in the example above, there are 11 syllables, thus there is one extra syllable at the end of the line. So, it is iambic pentameter with one extra syllable. This meter is used by poets for an extra effect, to call attention to something, because when you notice the extra syllable, you have to stop and think about it. It is a little like dissonance.
The lines above are from one of Shakespeare's sonnets and he makes use of this technique from time to time in his sonnets, that is why you saw this in a book about Shakespeare.
Read more about Shakespeare's sonnets here on eNotes.