Key Performance Indicators (KPIs): KPIs show the past, present, and future target (or planned) values.
What are the business issues concern you? What are the necessary answers do you need?
Such as business user would like to watch out Sales for the year of 2015 (this is a KPI). So we need to make an expression which will return the desired result. For example you have Dashboard where Year and Sales are the fields. So Here is the way to calculate Sales for the year of 2015.