Click New or Edit to edit the Rollup Definition.
The Rollup Definition screen has two main sections:
a) The step by step guide
b) Details - to check selections and/or overwrite
In most situations, there is no need to edit details.
When creating a rollup definition, there is no need to enter a name. If not provided, a meaningful name will be generated (TargetObject TargetField Aggregation SourceObject SourceField)
Step by Step Guide
... to be provided ...