Content and function words in sentence stress
Content words are usually nouns, verbs, adjectives, and sometimes adverbs. Those are the words that help us form a picture in our head; they give us the contents of our story and tell our listener where to focus his or her attention. We want our listener to be able to quickly grasp the main content of our story, so we make the content words easier to hear by bringing attention to them with added stress.
Function words are the words we use to make our sentences grammatically correct. Pronouns, determiners, and prepositions, and auxiliary verbs are examples of function words. If our function words are missing or used incorrectly, we are probably considered poor speakers of English, but our listener would probably still get the main idea of what we are saying. Since function words don't give us the main information, we don't usually want or need to do anything to give them added attention and the words remain unstressed. In addition, sometimes we do things to deliberately push function words into the background... almost the opposite of stressing. This is called reducing.
Not very many aspects of English are concrete, and the idea of stressing content words, but not function words, is a generalization and not a rule. Not every content word is said louder or longer, and not every function word is reduced. A speaker chooses exactly which words to stress based on the message he or she is trying to send.