Some languages, like French, have no stress, all the syllables are pronounced clearly and with the same strength. Some other languages, like Spanish, have a stress, that means that in every word, one syllable is pronounced with more strength than the rest (we call it "the stressed syllable") and the rest of the syllables are weaker, but they are all pronounced clearly. And then we have languages like English. English has stress, but the non-stressed syllables are not pronounced clearly, in fact, they are so weak that sometimes it is difficult to hear them if you are not used to it (they are very often pronounced with a Schwa: ). These videos will help you understand the English stress a bit better.
An English teacher talks about stress. A simple introduction.
This American teacher explains stress in more detail.