Word stress is the emphasis we place in a specific syllable of a word when pronouncing it. In English words that have more than one syllable, we usually don’t pronounce every syllable with the same weight, so each syllable in a word can be stressed or unstressed.
Stressed syllables are louder than the others – i.e. air comes out of our lungs with more power; but they might also be longer, or pronounced with higher or lower in pitch. Syllables that are not pronounced with such emphasis are usually referred to as unstressed syllables, and they are usually not pronounced as clearly as the others.