The two most likely possibilities have been mentioned.
If it must be done in InDesign, the button approach would be the most likely, although making the button field read-only would improve things even more, because then the user would not be tempted to click it (because the cursor won't change when viewing the document).
Using a sticky note has its advantages as well, however, one of the issues with sticky notes is that they can easily be moved around, and they require a specific setting in the Print dialog (do not print annotations).
If your form is used multiple times by the same user, you may have to be aware of the annoyance factor by showing the (obviously aimed at noobs) instructions. The workaround here would be to add a "Help" or "Info" button near the field, and in the MouseUp event you show the button or text field with the information, and in the MouseExit event you hide it again. This approach also helps you regaining real estate on your document, which otherwise would be used up by your instructions.
This approach has the advantage that it can deal with longer and/or content-rich explanations (you determine how big the field for displaying has to be). For short explanations, PDF fields have two additional possibilities to show such information; however you will need Acrobat to properly insert them:
a) Tooltip: If you open the Field Properties dialog (either in the Form Edit Mode, or using the Select Object tool (the big fat pointer icon), you have the Tooltip field in the General tab. The text entered in this field will display for a short time when you hover the mouse over the field (a behavior very common in webpages etc.). Also note that the Tooltip text is taken into account by assistive devices. This method works for any field type.
b) Default value: This method works for unformatted text fields only. You set the default value to the explanation text. Then you have to add some actions: In the OnFocus event you test whether its value is the default value, and if so clear the field, and in the OnBlur event you test whether the field's contents is blank, and if so set it to its default value. You may also play around with the textColor, for example to set it to a grey value, in order to make a visual hint that this is instructional.
Both methods do not need any real estate on the document.
Note that method b) requires a PDF viewer which understands Acrobat JavaScript (but if it does not, the default value can simply be overwritten (if the PDF viewer understands fields at all (which is a requirement for your form anyways)).
