If the descriptions were stored in your data, this might be a bit easier. However, you might be able to do this by using the methods outlined at the links below--its a bit of work to set these up, though, so you may want to weigh the pros/cons. It might be simpler to just create a hover button tooltip with descriptions of your measures. An example of this can be found here: Business AnalyticsTooltips can be used in Tableau to give instructions on dashboard use
Customizing/Conditional Formatting Tooltips links:
I hope this helps.