English has many informal words that should generally be avoided in more formal academic writing. Some examples are words like big, thing, stuff, and others that are perfectly fine in casual conversation when we don't have much time to think of the perfect word. However, in writing, we need to make sure we are specific and clear in our meaning, so vague and casual words like big, stuff, and good should be replaced by more accurate words like immense, item, or healthy.
For a list of several vague words that are overused and some suggested replacements, please see the linked handout: