What Is Salesforce Roll-Up Summary Fields?
Roll-Up Summary Fields in Salesforce summarize data of particular fields from a set of related detail records and automatically display the output on the master record. These can be used to display the sum, maximum, and minimum value of a field in a related object record, also the count of all the related child records. They are only available to master objects in a master-detail relationship.
After creating a Roll-Up Summary Field on an object, the master-detail relationship cannot be converted into a lookup relationship.
Roll-Up summary fields are read-only fields.