Pipeline Summaries can be used to quickly calculate things like the total or average deal size in each Stage, and in the Pipeline as a whole. Pipeline Summaries let you see an aggregate value of a Column on a group basis and over the entire Pipeline. The aggregate value can be chosen from a variety of functions such as "max", "min", "sum", "average", and "count".
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To adjust the Column Summary, hover over the Stage bar or navigate to Summarize from the dropdown.

To adjust the Pipeline Summary for the entire Pipeline, click the dropdown to the right of your Pipeline name and select from the options.