Say that you have a report with days going down the side and regions going across the top.
=Sum(Fact[Sales]) will calculate the "filtered result" for each cell in the pivot table. For example, the top left result cell is at the intersection of Region=Central and Date=1/1/2010. If you don't specify any filters, the asking for Sum(Fact[Sales]) will automatically include the filters inherent in that cell's position in the table, i.e. Region=Central, Date=1/1/2010.
Frequently, you will want to calculate a ratio where the denominator calculation needs to ignore the filters inherent in that cell. To look at all sales in the denominator, use:
Calculate(Fact[Sales],All(Fact)).
The All([table]) function in DAX will retreive all of the sales figures in the entire table, not just Region=Central, Date=1/1/2010.
Thus, the syntax for the measure will be:
=Sum(Fact[Sales])/Calculate(Fact[Sales],All(Fact))