In different places in Fetch you find the text field "Description".
1. Description of the Questionnaire (metadata, not displayed to the respondent)
2. Description for a main question
3. Description for a question part
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.
The description for main question can be used to organize several question parts with the same topic, while the description for a question part can contain specific information, or instructions, like, for example, “Select ONE of the options”.