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.

in photography, The main stress is on the second syllable (the "tog" part).photography = pho-TOG'-raph-y. The same as photographer; the stress is on the second syllable. This is distinct from "photograph", where the first syllable is the stressed one.