Summary columns are used for numerical columns in hierarchies. This number is calculated from the child items of a parent item and is not stored in the database. If the summary information is needed in JIRA you need to enable this column.
In the example below the "Estimated days" on "Main story" is not set, it is calculated from the sub-items.
Like the example above this column is used when numerical values are calculated on broken down items. Let's say we have the setup like in the screenshot below. "Main story" has an original value set in the "Estimated Ideal Days" column.
If we then indent the sub-tasks that value will be replaced with a new "Estimated Days" calculated from the sub-items. The old value of 2 days is however still stored in the Hansoft database and the calculated "Estimated Days" is presented by the client. In order to expose the "correct" value of "Estimated Ideal Days" in JIRA, the 0 value is stored in the "Stat" column.