Essay questions are designed to allow you to give a manually graded question on Quest assignments. These questions can be graded within Quest or have grades uploaded as described here: Importing Student Grades. Grades for assignments with essay questions will not be shown until the grades for the manually graded questions are provided.
Once you have finished filling in the content of the question, you will see the preview on the right hand side updated with the essay prompt.
After you are finished editing publish the question with the button in the upper right hand corner for use on assignments.