In general this means in an essay that you should avoid:
a. colloquial words and expressions; ""stuff", "a lot of", "thing", "sort of",
b. abbreviated forms: "can't", "doesn't", "shouldn't"
c. two word verbs: "put off", "bring up"
d. sub-headings, numbering and bullet-points in formal essays - but use them in reports.
e. asking questions.
Julie, I'm your practice teacher, you're my student. And I have to tell you to refrain from using terms such as "loony", "fruitcake" "nutcase" and "one sandwich short of a picnic" to describe people with mental illness. Oh, come on, Clare! Don't be so PC. Are you telling me you've never used those expressions? Well, ... yes ... but not in my dissertation.
Try the following exercises: 1. Colloquial words and expressions, 2. Abbreviations, 3. Two-word verbs, 4. Formal/Informal