The process of viewing numeric values or measures at higher and more summarized levels of the data is called aggregation.

When you place a measure on a shelf, Tableau automatically aggregates the data, usually by summing it.

You can easily determine the aggregation applied to a field because the function always appears in front of the field‘s name when it is placed on a shelf.

For example, Sales becomes SUM(Sales). 

You can aggregate measures using Tableau only for relational data sources. Multidimensional data sources contain aggregated data only. In Tableau, multidimensional data sources are supported only in Windows. (Source)

According to Tableau, Disaggregating your data allows you to view every row of the data source which can be useful when you are analyzing measures that you may want to use both independently and dependently in the view.

For example, you may be analyzing the results from a product satisfaction survey with the Age of participants along one axis.

You can aggregate the Age field to determine the average age of participants or disaggregate the data to determine at what age participants were most satisfied with the product.