Open-ended words such as "stuff" and "here" give your reader a general idea about something but don't define it. Think of vague words as category labels instead of specific words. For example, "they" suggests a group of more than one living thing but it tells your reader little else. The fact that "they" could refer to either "the girls" or "the bears" suggests that it is too vague unless you've written about "them" more specifically earlier in your paragraph. When you edit your work, ask yourself "who," "what," "where" and "when" questions to make sure that any suggestions of categories are defined in your writing.
Vague words leave the impression that you are either unfamiliar with your subject or there isn't anything interesting to say about it. Vague writing is, at best, boring because it lacks details that spark the readers' interest. In addition, people are much more likely to believe you if you write authoritatively, with specific nouns and verbs. Imagine how you would feel if your teacher said, "All that stuff will be on next week's thing," instead of a more specific sentence, "The periodic elements will be on next week's test."
Sentences with too many vague words are often longer than they need to be. School projects written this way appear unfocused and suggest to teachers that you have a very limited understanding of your subject matter. Consider the sentence, "The things that plants use to absorb what they need from the sun." Replacing "the things that plants use" with "chlorophyll" and "what they need" with "energy" gives you a shorter, more concise sentence that communicates your mastery of the subject.
Sometimes a writer assumes readers knows more than they do. Pronouns, such as "she" or "they" with no referent, and undefined directional terms like "over there" and "here," make readers think they missed something. "Who is she" or "where is here" are questions that stop a reader's train of thought and makes it difficult to follow what you are saying. Pronouns definitely have their place in writing, as too many proper names sounds stilted and unnatural, but because the reader can't stop you to ask for clarity, as a listener can in conversation, be sure to check that your pronouns and other vague words have clear referents earlier in your text.