In different places in Fetch, you can find the text field "Description".
- Questionnaire Description: This is metadata, which means it's not shown to the respondents.
- Main Question Description: You can use this to group or introduce several related questions.
- Question Part Description: This offers more specific details or directions for a particular question, like "Pick ONLY one option."
Since descriptions 2 and 3 are both optional, you can include, or exclude, either if you so wish. They are both displayed on the same page, so if you have a question only consisting of a single question part, then you probably only need to use one of them.