Take a look at tool tips. With tool tips, you can pop up a short text message whenever you hover the mouse over an object on the layout for which you have set up a tool tip. To create one, enter layout mode, select the layout option and then use the Appearance tab to set up the tool tip. The tool tip can use a calculation to include text from fields of the current record as a way to "customize" the tool tip message.
What you are describing is like a 'tooltip'. In layout mode click on the field and in the Inspector Palette, Appearance Tab, you can specify a tooltip (even by calculation).