An unstressed syllable is the part of the word that you don’t emphasize or accent, like the to– in today, or the –day in Sunday. An unstressed person is someone for whom every day feels like Sunday.
When you pronounce a word with multiple syllables, like avocado (to choose a random example), you put more pressure on some syllables than others — in this case, the ah and the cah. Consider those syllables stressed. If they were people, they’d be biting their nails and pulling out their hair while sitting at a desk piled with paperwork. The vo and the do, on the other hand, would be unstressed — lounging with their feet on the desk, doing crossword puzzles.