As you know, Tableau aggregates the Measure when you use it. By default, Tableau usually uses a sum. With the Aggregation default property, you can change that. For example, in the Sample Superstore Excel file, Discount is not a Measure that you can sum. You can change the default Aggregation to Average. 

Total using is quite similar; here, you can specify the aggregation used to display totals.

That's it for the default properties. These options are used to increase your speed in Tableau and are helpful if you want to share your Data Source. Another great way to customize your Data Source is to add hierarchies.