You can add descriptive help text either in the description of the question or as placeholder text in your question's configuration settings. Here's how:
Locate the question you'd like to add help text to in your form editor.
Below the text "What is your question?", locate the smaller line of text labeled "Add some help text".
Click on this text, and begin typing.
You're all set! When the form is loaded for your respondents, your help text will appear just below the title.
Note that placeholder text differs from a Default Answer, as any text entered here won't show up in your submission results.
That's it! Your help text is ready to be shown on the form.