When you add or edit an ad-hoc query and select a field under Result fields to display on the Select filter and output fields tab, the Summarize button enables you to view a calculation of the field results rather than the actual contents of the field.
Query is a way to group records; it is not a reporting tool. As such, the output fields identify records included in the query. If a field with a one-to-many relationship is selected under Include records where or Result fields to display, the record may appear multiple times in the query: once for each time the record satisfies the selected criteria. This allows you to check your query to ensure you get the expected results.
The Summarize button's functions - Count, Sum, Average, Min, Max - can help prevent these duplicates.
Some summary function scenarios include:
COUNT: For number or amount fields, the COUNT function is the total number of records included.
SUM: For number or amount fields, the SUM function is the combined total of the numbers.
AVG: For number or amount fields, the AVG function is the average of the numbers included.
MAX: For date fields, the MAX function displays the maximum or latest date. For number or amount fields, MAX displays the largest number.
MIN: For date fields, the MIN function displays the minimum or oldest date. For number or amount fields, MIN displays the lowest number.