Aggregate functions are special functions which summarize columns from lower levels. Aggregate functions can be used on on Grouped levels or in a Total column. For example, they can be used to Sum all of the values of a column, or Count all of the values for each grouping of a column. As such, aggregates must always refer to columns in a level lower than the result.
If the column being aggregated is grouped, then the aggregate is computed for each group. For example, if a column[Revenue] is grouped by year, then Sum([Revenue]) computes the revenue for each year.
The results of aggregate functions can be referenced by lower levels. An example would be to create a Sum([Amount]) column as Total and then an [Amount] / [Total] column as Percentage. This converts each Amount value into a relative percentage value.