stressed syllables are the syllables within a word that have the most emphasis when spoken.
for example:
other - the syllable "oth" is stressed and the syllable "er" is not because "er" is pronounced less that "oth"
the "er" tends to sound as if it were falling away at the end of the word.
compound words; tend to be double stressed because both syllables are pronounced equally
for example
childhood- both "child" and "hood" are pronounced fully
you can tell which syllable is stressed by saying the word naturally
In words of two syllables or more, at least one of the syllables is usually pronounced with extra emphasis. We can divide syllables into stressed and unstressed categories. 

impound; (words with prefix are usually stressed on the second syllable) this would be stressed as imPOUND

subject(verb); as a verb is stressed thus subJECT

canoe; is stressed on the second syllable as caNOE

knowledge; is a compound word made up of two different words ''know and ledge''. it is therefore stressed on the first syllable as KNOWledge